Bonus Boss

""Tyrant Overlord Baal? Wasn't he just a ploy to force players into playing their games for 100 hours longer?""

- Disgaea 3: Absence of Justice

Originally, video games had a much smaller (and more devoted) fandom. In recent years, the Demographics have greatly expanded.

What this means is that the general skill level has decreased away from Nintendo Hard, as it's no longer the hardcore devoted who are buying the products. This has upset said hardcore devoted quite a bit.

So, the solution has been the Bonus Boss (who often lives in the Bonus Dungeon). The Bonus Boss often exists outside the normal plot of the game, and requires quite a bit of conscious effort to get to. Its difficulty is usually much beyond that of the "story line" final boss. Occasionally, games may have more than one Bonus Boss. The key point here is that they make up the most difficult enemies in the game, and that includes the actual last boss of the game. Sometimes, the player may acquire some special ability or skill once they beat this extraordinarily difficult enemy, but it's usually just for bragging rights. Anyone who can beat the Bonus Boss has proved he doesn't need them.

A lot of the times, this is meant as a test of the player's skill. (Tetsuya Nomura has mentioned this as being the sole purpose of the Lingering Sentiment in Kingdom Hearts II Final Mix. Then, it turned out his pants were practically ablaze.)

A Bonus Boss can technically be a Giant Space Flea From Nowhere, but then again, you went looking for it.

Compare True Final Boss, which is often (but not always) a Bonus Boss that, when found or unlocked, ends up replacing or coming right after the Final Boss, requiring you to beat it in order to clear the game.

A Bonus Boss that drops useful items may lead to Unstable Equilibrium.

Action Adventure Games

 * The Four Sword Links in the The Legend of Zelda a Link To T He Past port on the Game Boy Advance. Each Link had some abilities that Link could use, such as the Hurricane Spin, the Magic Cape, etc. Beating them was purely for bragging rights.
 * The Updated Rerelease of The Legend of Zelda Links Awakening for Game Boy Color featured a Bonus Dungeon based on color. The boss of the dungeon wasn't more difficult than the other bosses, but could only be defeated by knowing the color of his clothing and attacks.
 * Castlevania: Order of Ecclesia gave you the option to fight the Chinese vampire, Jiang Shi, in the Large Cavern bonus dungeon. He's not very hard though, seeing as how proper use of Melio Scutum and any slashing Glyph would easily reduce his 6000+ HP down to nothing. Though this boss is interesting in the fact that when he dies, a seal is placed on his face, but if you break it off with an attack, he comes back to life, allowing you to fight him again as many times as you want. Not worth the attribute points though (30, 60, or 120)
 * Galamoth in Castlevania: Symphony of the Night has more HP than Dracula and hits really, really hard. Fortunately, you don't have to beat him to meet the Count.
 * Of course, if you are equipped with the right item (hidden away, naturally), Galamoth becomes a joke, as most of his attacks will just heal you instead. But God help you if you meet him in Richter Mode...
 * Maria mode in the PSP version... Your attacks do 1 damage if not aimed at the head, which is off of the screen while on the ground.
 * Beezlebub is also optional, as are a few other bosses in the inverted castle. Beezlebub is the only one that's remotely challenging, though; the rest just go down as quickly as most other bosses by this point (ie. within 30 seconds).
 * Legion, Nucleolous, and Golden Bones in Castlevania: Curse of Darkness. The former are more hidden than ultra-difficult, but the last...
 * The last isn't a boss so much as it is a Metal Slime. A very, very sturdy and strong Metal Slime.
 * The Whip's Memory, an image of Richter Belmont in Castlevania: Portrait of Ruin, is part of a ritual to unlock the true power of a weakened Vampire Killer whip. Unlike the rest of the game, you can only battle this boss with Jonathan. Thankfully, "dying" in this battle just boots you out of the battle with full HP and MP instead of yielding a Game Over.
 * The entire Nest of Evil with its plethora of borrowed bosses and new enemies.
 * Castlevania: Lament of Innocence has The Forgotten One, a giant, skinless demon trapped in the Prison of Eternal Torture, an area accessible when you open the Hub Level. (You still need a late-game artifact to fight him, but the area is open.) Everything about him is terrifying, and he's pretty difficult.
 * Illusion of Gaia has Solid Arm, a boss originally from the first game in the series, Soul Blazer, who's only fightable if you collect all fifty Red Jewels.
 * The Legend of Zelda Spirit Tracks has at the end of Take Em All On's Final Level.
 * Aquaria has a number of optional bosses, but many consider Simon Says to be the most interesting. He's well hidden, and you don't actually fight him - instead you play, well, simon says, with a very useful third cooking slot as your reward for playing well.
 * Okami has a number of these. The prize for beating them is usually a Stray Bead.

Action Games

 * Metroid: Other M brings us the Metroid series's first bonus boss. If you play the post-credits sequence for 100% and the extra ending, you have to butt heads with Phantoon from Super Metroid, who not only has new powers, but is also much scarier looking than before.
 * It is worth noting that the Phantoon fight was intentionally left out of Hard Mode.
 * Cave Story has Ballos, who was entombed within the island many years ago, and whose magic is causing the island to fall; the Heavy Press, who comes before Ballos; Ma Pignon, who will restore Curly's memory; and the Red Demon, who you find at the end of the hidden Final Cave.
 * Though, Ballos counts more as an example of True Final Boss, and beating the others is necessary to get to him.

Beat Em Ups

 * The Red Dragon also features as a Bonus Boss in Tower of Doom and Shadow Over Mystara, the Capcom Beat Em Ups based on Dungeons and Dragons.
 * Rodin from Bayonetta can be fought if the player buys a Platinum Ticket for 99,999,999 halos. He is revealed to be a former Lumen Sage, and is a thousand times more powerful than Jubileus. Each of his attacks take off three quarters of Bayonetta's health, and one in particular is an instant-kill.
 * Benny and Clyde are the only bosses in River City Ransom that you don't have to defeat in order to access River City High School. Once inside, you can also skip Tex as well, since only Otis has to be defeated in order to fight the Dragon Twins, and then Slick.
 * Via Asura's Wrath DLC, Two Bonus Bosses that have nothing to do with the main game storr are Evil Ryu and Oni.

Fighting Games

 * Mortal Kombat was arguably the Trope Maker, with the first game having Reptile as a secret boss. There is a very small chance that he will appear before the fight begins and give you clues on how to find him (ex."Look To La Luna"). You can only fight him if, on the fight at the Pit stage, you don't block, get a Double Flawless on your opponent, and finish them with a Fatality. Furthermore, there must be a ghost floating in front of the moon. If the conditions are met, the screen will flash with the words "You have found me, now prove yourself!" appearing, whereupon you will be taken to the Bottom of the Pit to fight him.
 * Mortal Kombat 2 continued the tradition with Noob Saibot, Jade, and Smoke. Noob Saibot appears only if you win 50 battles consecutively. Jade appears if, on the fight before the question-mark box, you only use the Low Kick button to defeat your opponent (can be done on any round). Smoke is arguably the most difficult to get, as you have to make Dan Forden appear and say "TOASTY!" while fighting on the Portal stage, then hitting Down + Start while he's on the screen.
 * Mortal Kombat 3 had Smoke as a hidden boss via one of the 11 hidden treasures of Shao Kahn that you can access after beating the game, as well as Noob Saibot. Ultimate Mortal Kombat 3 added Mileena, Ermac, Classic Sub-Zero, and Human Smoke. Mortal Kombat Trilogy added Chameleon and Khameleon, who were basically all the male and female ninjas rolled into one, respectively. You can also fight every single one of these characters via the Kombat Kode system.
 * Akuma from the Street Fighter series is usually a bonus boss in the games where he isn't the True Final Boss, particularly in Street Fighter Alpha 2 (as Shin Akuma) and in X Men Children of the Atom. Even if you're playing as a character whose final boss is supposed to be Akuma (like Ryu or Gen), you will simply fight Shin Akuma first and then you'll face the regular version of him.
 * The CPU version of Dan also served this role in the first Street Fighter Alpha, as did Q in Street Fighter III: 3rd Strike.
 * In Street Fighter Alpha 2 and Street Fighter III: 2nd Impact, each character has a specific rival who can only be fought if the player manages to fulfill certain conditions without losing a single match.
 * Street Fighter IV's usual final stage is Seth. Should the player play as him in the first costume, a red Seth will challenge the player.
 * Baiken, in the original Guilty Gear for PS 1. In order to get to her, you had to beat the game as Sol or Ky, without using any continues - once you get to her, though, you can try to win as many times as you like, and if you do, you unlock her as a playable character. Considering the extreme difficulty of defeating Justice the first time, and the first game's broken instant-kill mechanics, this was quite a task beyond the calling of most casual players.
 * Fatal Fury: Geese Howard in Real Bout Special. He dies in the first Real Bout, only to come back as "Nightmare Geese" in Real Bout Special (it's implied that the Nightmare Geese battle takes place in the player character's mind, as Geese has an odd aura around his feet and enhanced powers).
 * Much earlier in the series, Fatal Fury Special had a Bonus Boss in Ryo Sakazaki, who was the first seed in the long running franchise known as The King of Fighters.
 * Finally, Alfred Airhawk fills this role in Real Bout 2.
 * Giant Kirby, from Super Smash Bros. Melee.
 * ...and Giga Bowser (which is more of an example of True Final Boss).
 * In both Melee and Brawl, we have Crazy Hand, Master Hand's south-pawed counterpart, who will appear alongside Master Hand (after Master Hand reaches half health or at the beginning of the fight in Melee and Brawl respectively) if certain conditions are met while playing Classic Mode. Aside from there being two bosses to defeat instead of one, they are able to perform coordinated attacks that are more dangerous than what they could do individually.
 * Soul Calibur 3 also has a bonus boss. The arcade and Tales Of Souls modes will almost always be fought against Zasalamel's demonic form, Abyss, but an even stronger opponent called Night Terror can be fought. Night Terror will replace Abyss if the player encounters and defeats Olcadan before arriving at the cathedral where the last few battles take place. Night Terror can also be fought in the 'Final Battle' mission of the game's challenge mode.
 * The Virtual On series has several of these, some exclusive to certain versions. In the first game, if you win all your battles by Time Over, you'll get a Warning message and then enter a special battle with Jaguarandi, a mutated version of Raiden that varies each time you fight it. When you first fight it, it's about the size of a Raiden or a Belgdor, and its color is the opposite color of your mech. If you continue against it, it changes to a miniature toy-sized version that is colored purple, and its armor weakens severely (so much so that a close range attack from Apharmd will kill it instantly), and if you continue again, it can be killed in 3 hits by Temjin's Beam Rifle! In the Japan-only PlayStation 2 version, if you beat all 8 virtuaroids in under 30 seconds, you will hear a special chime and fight the original Fei-Yen instead of Z-Gradt. This version of Fei-Yen is always in Hyper Mode, but its armor is not weakened and its attacks are more powerful than the regular Fei-Yen.
 * The sequel, Oratorio Tangram, has Ajim - a crystalline, transparent virtuaroid. To fight him, you must win at least one battle in your run with a Time Over. Once this is done, he will appear randomly at any time, even as late as the Raiden fight, crashing down on your opponent and destroying them. His stats are beyond mortal comprehension, meaning that he's faster and stronger than all other virtuaroids, and all damage done to it is lessened to roughly 5/8 of the usual. Oh, and you can't beat him via Time Over either - if you try and do this, you will lose automatically. Encountering him automatically unlocks him for regular play in the later revisions, but on the Dreamcast version 5.45, he was only playable by beating him, then beating the game, and holding both Turbo buttons on the Random select box on the latter half of each month. And when you play as him, he has the weakest armor in the game, and to make matters worse, his health drains by 2% every second.
 * FORCE, the 3rd installment, has Shadow versions of your team that you can fight by, once again, getting Time Overs. These Shadow mechs are manifestations of a virus in the system, taking the forms of your mechs and making them much stronger than the normal version. Jaguarandi also returns, but as the default mid-boss, and this time it becomes LUDICROUSLY HUGE, taking up over half of the arena. It's also a mutated Guarayakha now instead of a Raiden, trading in most of its long range advantages for godlike close range combat tactics. Ajim also appears, with a female version of it called Guerlain, but as the final boss, and utilizing completely different moves than before.
 * Both Galaxy Fight and Waku Waku 7, being made by SUNSOFT, shared a common bonus boss named "Bonus-Kun", a punching bag (from "the Punching Bag Planet") with a limited moveset who parodies Ryu. In Galaxy Fight, after you defeat Felden, you get to fight a final Bonus Boss in the form of Rouwe, an old man dressed in a karate gi.
 * Samurai Shodown 2 had a very tough Bonus Boss in Kuroko, whom you could either fight by meeting certain conditions, or just randomly after stage 3.
 * One Must Fall has a few.
 * In the One Player story mode, there's Fire and Ice. To take on Fire, you need to be playing on at least the hardest non-hidden difficulty, beat your opponent on the Fire Pit arena, do a destruction on them, and enter a robot-specific code. If playing on the very hardest difficulty, beat Fire and do the destruction and code again and you can fight Ice. These two are extremely hard to beat, but the devs reward you generously for doing so. Normally, the end of round bonuses on the hardest difficulty are 400,000 points. For Fire, they're 2 million points, and for Ice they're 20 million points. Good luck getting a perfect round against Ice. The only way to not take any damage from him is to not take any hits at all, as he can still damage you even when blocking.
 * In the tournament mode, there are various unranked challengers in each tournament. All of them are hard, and most require you to be playing on the hardest difficulty. If they're going to show up, they'll challenge you after doing a destruction on some other opponent. There's at least one occasion when one unranked challenger will challenge you after beating another unranked challenger, too.
 * The obscure 90s fighting game Blood Storm had several of these, all of them Palette Swaps of your main characters. Unlocking most of them was both fun and ridiculously frustrating. For example, one of them required you to knock down a stalactite in one stage using projectiles, so that it falls down a pit. Then, you have to jump down, land on the small platform, and then you'll be able to fight the boss. Another required you to drop your weapon, and then perform the "pick up" command so that you touch the center of a summoning circle. And so on.
 * The final boss of the story mode in Dissidia Final Fantasy is Chaos, who is only level 50 or so and thus can be defeated easily with Level Grinding. However, beating him opens an extra campaign whose final boss is a level 110 Chaos called Ultimate Chaos.
 * Duodecim does the same thing, replacing the more-than-max-level Chaos with Chaos' new more-than-max-level One-Winged Angel, Feral Chaos. Your reward for beating him? The ability to play him.
 * In Tekken 6, there's, uh, Nancy. She's basically one of several giant robots developed by the Mishima Zaibatsu, perhaps to combat Azazel, who is finally free after 10,000 years. Her appearance is sort of a Big Lipped Alligator Moment in that you get rewards for beating her but if you don't, it has absolutely no bearing on your progress and you don't get the option of fighting her again without restarting.
 * Lightning Legend Daigo no Daibouken has two of them, Girigiri Oyaji (the game's Mr. Exposition and sponsor of the All There in the Manual national fighting tournament) and K.O.J. (the current champion of said tournament). You'll fight them after the Final Boss, if you have finished the game with all regular characters, then did a No Continue Run (and for K.O.J., you need on top of that to gather one hour's worth of matches) beforehand.
 * In Blaz Blue the only way to fight Unlimited Ragna is to go through Score Attack Mode or to play through arcade mode finishing each opponent with a distortion finish.
 * In Persona 4 Arena, it's possible to unlock a special bonus match against She is an SNK Boss to the core, can inflict multiple status effects, can heal herself, and is insanely difficult to kill. If she is in a position where she can win the match, she'll just perform her One-Hit Kill attack on you and be done with it. Of course... considering she is one of the most famous Bonus Bosses of the modern era, this is all to be expected.

First-Person Shooter

 * Borderlands has some in the optional side quests, some of which, like Mothrakk and Marley and Moe, can reach That One Boss status.
 * The true Bonus Boss doesn't come until you've downloaded and finished The Secret Armory of General Knoxx. That unlocks a fight against Crawmerax the Invincible, which is pretty much Exactly What It Says on the Tin.

Hack and Slash

 * Versions 1.00-1.09 of Diablo II had the Cow King as a sort of Bonus Boss, though he was substantially weaker than the actual final bosses. Version 1.10 introduced Uber Diablo as a new Bonus Boss, and 1.11 introduced the Pandemonium Quest and Uber Tristram.
 * Dungeon Siege has two. One is easy to find, while the second is extremely hard. The first is Scorch, the ancient dragon of Rathe, whom the player could avoid simply by continuing by the road to Castle Ehb. Scorch is the biggest monster in the game, has the highest number of HP, and is nearly as deadly as the final boss. The second Bonus Boss is located in the hard-to-find secret "Chicken level", amongst the various chickens named after the game's developers. It is.
 * Henry in No More Heroes.
 * In the Sequel to Otogi: Myth of Demons the last bonus mission in the Forest of Havoc is a dual with The Crimson King, a recuring character from the first game. How hard is he? Lets list the ways. He hits like an angry truck, he can move just as fast as you, he throws magic fireballs that cause a bad status effect, and his ultimate attack hurts like hell...did I mention that it's a Homing Projectile? If you had trouble beating the last boss of the game then The Crimson King will wreck you without mercy.

MMORPGs
"You don't understand the math behind it, but you're pretty sure you're equal to or greater than eight of yourself."
 * Most MMORPGs have such things: Giant Monsters in City of Heroes, world/raid bosses in Everquest, and raid bosses in World of Warcraft. Some are 'storyline' bosses, of course, but a lot of them are easily skippable.
 * Guild Wars features a few of them, most notably Urgoz, Kannaxi, Mallyx, Duncan the Black, and most recently, Dhuum. To make matters worse, all of them (save for perhaps Duncan) have a hard dungeon to finish in one sitting before you can face them; if you die, you have to do it all over again! The placement of Mallyx and Duncan make them candidates for True Final Boss.
 * zOMG features the Landshark, which appears semi-randomly in Gold Beach. The Landshark is one of the only charge level 10 (max CL) monsters in the game, and features insane attack power, extremely high HP, and attacks that can take up almost the entire screen. If you see one, running is advised.
 * A recent update nerfed the Landshark to more manageable levels. (It's now only CL 7). However, a gamut of new bonus bosses were added in its stead, as well as "Challenge" versions of all Instance Bosses. While none of these (With the exception of ) are CL 10, they have the added difficulty of only being vulnerable to individuals below a certain charge level. Meaning that players wishing to battle Airshark need to suppress their CL to level 2.
 * World of Warcraft also has its share of bonus bosses. The first one was Nightbane in Karazhan, where the only way to access him was to complete a certain quest chain. Wrath of the Lich King also introduced bonus bosses in some of the regular dungeons that are only accessible on Heroic difficulty. However, the WotLK bonus bosses tend to be much easier than the the Final Boss of their respective dungeons.
 * Just in case that isn't enough (likely, given how hardcore some fans of the game can be), many of these same bosses have special conditions that can be attained for Achievements (a Bragging Rights Reward, since you can't do anything with the points you accumulate from achievements), which often turn fairly normal bosses into insanity. For example, one of these takes place in a dungeon where there are minibosses that you can kill to make the main boss easier; for the achievements, you have to leave at least one of the minibosses alive until you engage the main boss. For the hardest achievement, all three get to stay up, and they come help the main boss while you're fighting him.
 * Yogg-Saron, the final boss of Ulduar, has a similar hard mode: you are required to defeat his four jailers, the Watchers (whom he's driven mad; defeating them restores their sanity) in order to open the path to Yogg's chamber. If you ask, they'll help you in the fight against Yogg-Saron. The achievements for defeating Yogg-Saron take the form "X Lights in the Darkness", where X is the number of jailers you ask to help you. Asking none of the jailers to help you is referred to as "Yogg+ 0", and is - even two raids later - the hardest fight in the game.
 * The Temple of Atal'Hakkar (aka Sunken Temple) has the first bonus boss in the game: The Avatar of Hakkar. You need to complete a quest chain to access him, too, and he's actually harder than the Dungeon's final boss, Eranikus the Dreamer.
 * The best example, however, is Algalon the Observer (a.k.a. Algalon the Raid Destroyer) in the Ulduar raid. He is only accessible if you complete a quest that requires the player to kill several bosses in hard mode, and if he's not killed within an hour or so from summoning him, he'll despawn and you have to wait till the raid reset to summon him again. Algalon was not the hardest boss in the game when he was released - that honor went with Yogg-Saron on full hard mode; see above - but he was the one that the fewest raids defeated, between the difficulty of the path to get to him and the difficulty of the fight itself. (As a special bonus, story-wise, if you fail to defeat Algalon, he sends a signal to his superiors to begin sterilizing the planet to rid it of corruption so that life can begin anew.)
 * Jin'do the Hexer in Zul'Gurub was optional; you can actually skip him, and he drops some of the best loot in the dungeon next to Hakkar. Two other bosses were accessible through skill-related means; it was possible to fish up Gahr'zahka by catching fish to make a lure for him, and preparing a certain kind of mojo with Alchemy enabled raids to fight bosses at the Edge of Madness.
 * In the re-released Level 85 Heroic Zul'Gurub, people with enough Archaeology skill can access the optional boss at the Edge of Madness.
 * Bug trio, Viscidus, and Ouro were all optional bosses in Temple of Ahn'Qiraj. Most, if not all guilds, did bug trio because the fight wasn't very difficult and it rewarded good loot. Viscidus is a fight that many guilds skipped because, even at 80, it's still a pain in the ass. To defeat Viscidus, he must be frozen; naturally, he can only be frozen by Frost-based attacks, such as Mages' Frostbolt or Shamans' Frost Shock. Once Viscidus is frozen, then everyone in the raid must melee him (yes, even you healers need to). If melee'd enough times, he will shatter. All while attempting to freeze and shatter Viscidus, the raid must survive near-constant AoE Poison damage. Ouro is a fight that most guilds would skip in favor of C'Thun because A) he offered better loot and B) killing C'Thun at 60 was a significant achievement for raiding guilds.
 * A number of dungeons from Wrath of the Lich King have a boss that only appears when running the dungeon in Heroic mode. Eck in Gundrak is a good example, as not only does he only appear on heroic, but also appears in a side alcove that opens up after Moorabi is defeated on Heroic.
 * Recently introduced with the Cataclysm expansion is Lady Sinestra. Only accessible in 10 and 25 man raids, AFTER clearing the entirety of the Bastion of Twilight in Heroic mode.
 * In the Firelands, Ragnaros has an entirely new fourth phase on Heroic mode.
 * City of Heroes has the Army of Me badge, unlocked via a special option in a Villain Side story arc. The mission? Beat a full team of yourself.


 * Final Fantasy XI has the Notorious Monsters. The most, well, notorious of these are the Pandaemonium Warden, a Sequential Boss with 10 different forms who has to be beaten in 2 hours, and Absolute Virtue, who was originally designed as a Hopeless Boss Fight. Allegedly, they revamped him to be beatable, but other than exploiting bugs, nobody has yet beaten him. That's right: in an online game with over a million players, no group has even once managed to overcome Absolute Virtue.
 * He has now been beaten legitimately. It happened within two months of the Level Cap being raised to 80, but he is still nearly impossible.
 * He was beaten legitimately before by an alliance of Scholars stacking Modus Veritas, cue Modus Veritas getting a monster nerf to the point its a waste of time and merits.
 * Kingdom of Loathing has the three final bosses of the Clan Dungeons.
 * Mother Slime in the Slime Tube, who can be up to level 8700. She can also gain immunities to elemental damage and is immune to stunning moves and items.
 * The Necbromancer, who has three forms and a special attack that takes 30% of your HP.
 * Hodgman the Hoboverlord, who has 25000 HP, gains buffs depending on whether his dragons are defeated or not, and is one of the few enemies that can flip out unprovoked. Defeating him can get you an imaginary hamster.
 * Dragon Nest has several high level maps feature alternative routes with different bosses.

Platform Games

 * Mega Man X 6 had some bonus bosses by going through alternate routes to fight Zero Nightmare, which nets you Zero, and High Max, which lets you skip straight to the last areas of the game, although it's hard to find out how the hell you're supposed to even damage him. (Hint: .) Going to the secret areas again will let you fight Dynamo, which lets you get large amounts of souls.
 * X8 had Cut Man, again. You need to go through Optis Sunflower's stage to reach a 3D-ified version of where you fought him in Mega Man.
 * X3 had Vile MK-2, which does nothing when defeated, except when his weakness (Ray Splasher or Spinning Blade) is used to score the final hit, in which case you won't fight him later. This is only the first step in getting the Infinity+1 Sword. There was also Bit and Byte, who, like the X-Hunter example below, you could find and battle in hidden areas in stages. If you beat them using their weaknesses to score the final hit (Frost Shield or Triad Thunder for Bit and Tornado Fang or Ray Splasher for Byte), then they would be replaced in the final stage by Press Disposer. If even one of them was left alive, however, then the player would fight Godkarmachine O Inary in the final stage. Also, if Vile was defeated, the player would fight Volt Kurageil and Mosquitus in the final stage instead - defeating Mosquitus with Zero is the requirement for X to obtain the Z-Saber.
 * X2 has the X-Hunters, Serges, Violen and Agile, who can be encountered in hidden areas in stages. If you beat them at the first opportunity, you receive parts of Zero, and collecting all of them enables you to skip the fight with Zero in the final stage. You still fight the X-Hunters in the final stages, regardless of whether you fought them before or not, though.
 * In Mega Man Zero 3, in a secret area in the second-to-last level that can only be opened by going into Cyberspace, Zero will encounter Phantom, who died two games earlier, ready to fight him to test his worth as a hero. Victory will net the player the infinity plus one, er, boots.
 * Mega Man:
 * Mega Man 8 had two in the Sega Saturn version, and they happen to be Cut Man and Wood Man from the first two games, respectively. Unlike their original appearance, however, they only give you bolts to buy weapons with. Cut Man is hidden in Duo's stage halfway through the game. Wood Man doesn't play the trope straight, though, as you fight him right before the continue point in Search Man's stage.
 * 10 gives you the option to buy three extra bosses as Downloadable Content. They show up only in Time Trial mode, though, but they give Mega Man three very useful weapons only he can use.
 * ROM Hack Rock Man 4 Minus Infinity has Crash Man and Wave Man. Finding their secret locations and beating them allowed Mega Man to use the Wire and Balloon Adapters before the Cossack Castle stages.
 * In Mega Man ZX, there is a similar type of battle. I forget the details, but near the end of the game, you can find This battle is noteworthy for actually being HARD, not just "OMG he's level 500", since this time your character has far fewer abilties.  AI has also become much more aggressive since the last game, and can defeat you in literally seconds if you're not quick with the fingers. Beating him gives you  Only not really, since you only get it after beating the game and it's not really that much better then Model ZX.
 * Ratchet and Clank Future A Crack In Time has
 * And also in Ratchet and Clank Going Commando, there's a secret boss in Oozla, the Swamp Monster II. It's harder than most other bosses in the game and also rewards you with the Box Breaker after killing it. You can also only get there if you have the Gravity Boots, which you get much later on in the game. You will also probably need the game's most powerful weapons to defeat it.
 * The boss of the Engine Room in Super Mario Galaxy is technically a Bonus Boss, because getting to the final boss only requires unlocking the Engine Room and collecting 60 stars (from anywhere). Of course, it is required to beat this boss for Hundred-Percent Completion, as beating it opens up the last few levels.
 * Dynamite Headdy has a Bonus Boss called "The Money" that is unlockable with a password you get by beating the bonus game four times.
 * La-Mulana 's infamous Hell Temple has its own guardian, The Boss (no, not that one ), a giant blob with Naramura's (the game ideator) signature face that behaves a lot like the 4th boss of Maze Of Galious and summons smaller copies of himself that throw grenades around. Interestingly enough, The Boss is generally easy compared to the level around him.
 * The Legend of Spyro: Dawn of the Dragon has Elite Enemies in every chapter. They look like normal enemies with a few modifications. They're not. They're hard to damage, let alone kill...

Puzzle Games

 * In Professor Layton and the Unwound Future, if you go to the Secret Content and submit a code saved from the second game, you get to do a bonus puzzle designed around the MacGuffin of the second game, and apparently designed by the second game's villain. It's an incredibly hard puzzle, and the villain refuses to let you use Hint Coins, as he wants you to beat this puzzle in its purest form.
 * All three games in the DS series have a series of puzzles that are unlocked by way of completing the various side quests (for example, completing the mechanical dog in the first game or all the toy car courses in the third). Completing all those puzzles unlocks a final set of puzzles that are the most difficult in the game.

Racing Games

 * In any version of Initial D Arcade Stage, defeating every opponent in Legend Of The Streets mode will take you to a free battle with Bunta Fujiwara. He is extremely difficult to beat, often sporting Rubber Band AI. Whether or not you defeat him, the credits will roll afterwards.

Real-Time Strategy

 * The Smokey Progg in Pikmin is one of these, as you have to go out of your way to find it and it drops an object that produces 100 Pikmin when it dies. It has the difficulty to match as well: just touching its slime trail can kill several Pikmin at once, and any Pikmin attacking the Progg will probably be thrown right into it.
 * The enemy fountain from Defense of the Ancients. It will fuck up anyone who goes in unprepared, and its destruction is completely unnecessary.
 * Dawn of War II has two optional missions. One, against the Eldar, has you fight an avatar of their war god, which is considered to the hardest mission the the game, not just because the avatar is really strong, but because the damn thing calls in lots of vehicles for help. The other has you fight an Ork Warboss, who, while weaker then the avatar, is still considered tougher then the final boss, though part of that is clearing either of the levels gives you a nice set of terminator armor and because in the final level, you get to use all your entire squad, unlike the rest of the game.

Rhythm Games

 * While not a "boss" in the traditional sense of the word, this bonus level from Guitar Hero III, featuring Dragon Force (video game)'s "Through The Fire And Flames", may be harder to beat than any of the above examples.
 * And let's not forget its predecessor: "Jordan"" from Guitar Hero II.
 * And its spiritual successor "Visions" from Rock Band 2, utterly punishing on both guitar and drums.
 * From DJMAX Technika:
 * In the Specialist set, getting 70% accuracy or lower nets you Fermion SP, a chart riddled with Fake Difficulty due to sensor bugs when dealing with repeat (purple) notes. What's also agonizing about this is that if you're good enough to even unlock the set (via Special Set 6, where you need to pass the dreaded Son of Sun SP), you'll probably have to intentionally Do Well, But Not Perfect to unlock Fermion SP, so getting there in the first place will leave your HP at a very, very bad value.
 * In the Conqueror set, there will be a small chance that, instead of the usual boss charts, you'll get Thor TP; given that getting this song is excruciatingly random (or perhaps just insanely difficult, considering the theory that you'll have to get EXACTLY 98% or 94% accuracy), one does not have many chances to practice this song and will have to rely on YouTube videos. A hacked mission in Platinum Crew (in celebration of the Mid-Autumn Festival) spat up a chance to practice it constantly until it was removed, presumably after Pentavision's warning to the Chinese Plat Crew management.
 * In Technika 2, the Super Speed set brings us D2, a song running at 356 BPM (approximately 1.5 times faster than Son of Sun)... not to mention that to unlock this song, you'll have to play a very, very buggy song called BEE-U-TIFUL, which is very likely to leave you in a worse state than if you had tried to unlock Fermion SP in Technika 1.
 * Bemani games in general usually have an unlockable Extra Stage with one or more "boss" songs, which are always among the hardest in the game. Do well enough on that and you'll get to play the True Final Boss, usually named either "One More Extra Stage" or "Encore Extra Stage", and is generally the hardest song in the game (and gets progressively harder with each subsequent installment).

Roguelike

 * ADOM has quite a lot of these. Most of them reside in a Brutal Bonus Level and carry some artifact (indestructible powerful unique item) that drops when killed.
 * There's Rehetep, an undead mummy lord, who "lives" in a pyramid filled with traps and maze-like corridors. The pyramid is impossible to enter until the player character hits level 13, when an invitation from Rehetep will magically appear. The reward for killing him is the Ancient Mummy Wrapping, which grants several very useful resistances and passive abilities.
 * There's the Minotaur Emperor in the minotaur maze under a ruined city. He carries an axe that deals massive damage, but is also massively heavy.
 * Then there's also the blue wyrm Srraxxarrakex, extremely fast quickling bard Filk, the complete opposite Emperor Moloch, and Keriax the multi-headed chaos dragon, who are all necessary to beat in order to reach the True Final Boss.

Role Playing Games

 * Chrono Trigger DS had an Alternate Final Boss, the Dream Devourer, which has ties to the final boss of Chrono Cross, the Time Devourer. It is unlocked after you clear all three of the Bonus Dungeons that appear after you defeat the normal Final Boss.
 * Before that, there's the Dimensional Vortex fights against the Alabaster Shade, Crimson Shade, Steel Shade, and
 * The Lost Sanctum also has some, like the multiple fights with the Nu Guardian.
 * The ultimate form of Spekkio is also one of the strongest bosses in the game. But since you need maxed out characters to even challenge him there is not much strategy involved anymore...
 * The bounty bosses in Skies of Arcadia Legends. They are also That One Boss - because even for a bonus boss, their difficulty is sadistic.
 * However the Rewards some of them leave make the rest of the game piss easy. Captain's Hat + Berzerker Mail anyone?
 * Ragu o Ragula in all of the Wild Arms games for PlayStation, PlayStation 2, and PSP. Angolmois also appears in some of them. In fact, the Wild ARMs games have many bonus bosses, often found sealed in crystals found throughout the game. Ragu o Ragula is neatly incorporated into all of these titles as the sleeping demon who is fated to destroy Filgaia, centuries after the conflict-of-the-day is finished off. Big extra credit for overachieving heroes.
 * Gabriel Celesta and the Iseria/Isis/Ethereal Queen in Valkyrie Profile, the Star Ocean series, Radiata Stories, and Infinite Undiscovery...most tri-Ace games in general.
 * Freya from Valkyrie Profile is the strongest boss in Star Ocean: Till the End of Time where she makes a cameo appearence along with Lenneth.
 * They exist in pretty much every Final Fantasy game:
 * Ruby Weapon and Emerald Weapon in the international version of Final Fantasy VII.
 * Crisis Core has Minerva. Genesis's best form is a pussy compared to her.
 * Ultima and Omega, or Ultima Weapon and Omega Weapon, appear in many of the games, including Final Fantasy V, Final Fantasy VI, Final Fantasy VIII, Final Fantasy X, Final Fantasy X-2, Final Fantasy XI, Final Fantasy XII. Omega also features prominently in Dirge of Cerberus. Ultima was a mandatory boss in Final Fantasy VII as well as Final Fantasy VI, under the name Atma Weapon, which is a stupid mistranslation. Omega and Ultima are storyline bosses in XI's Chains of Promathia expansion, and are Sequential Bosses to boot.
 * The Monster Arena monsters in Final Fantasy X, and the Dark Aeons and Penance in the International version. Some of the Dark Aeons were required encounters if you want to backtrack to certain areas. Penance is entirely optional, and he's accordingly brutal... 12 million HP, potent attacks that devastate an unprepared party, and supporting limbs that do even more fun damage.
 * The Via Infinito monsters, led by Paragon and Trema, in Final Fantasy X-2, which are rather nasty for causing status effects even if you're immune to them.
 * Hell Wyrm, Yiazmat, Behemoth King, the eight non-story line Espers (which includes Zodiark), Omega Mk. XII, etc. in Final Fantasy XII. To date, Yiazmat is the boss with most HP in all of Final Fantasy (a grand total of 50 million), and can take multiple rounds to defeat.
 * Final Fantasy Tactics A2 had Upsilons A1 (leveled somewhere in the 60s) and A2 (Level 96) and the Level 99 mage clan.
 * Final Fantasy I had a special secret random encounter boss that was only able to be encountered on the walkway heading towards Tiamat. The boss was a mecha named Warmech and had damage capabilities surpassing Tiamat.
 * The remake in Dawn of Souls for the GBA included optional dungeons with bosses from III, IV, V, and VI, although only Omega and Shinryu from V were much harder than the regular Final Boss.
 * The PSP remake ups the ante with Chronodia, who has 8 FORMS! Which one you fight depends on how you did in the Bonus Dungeon prior to fighting her. Either way, she's pretty hard.
 * Final Fantasy IV had four optional boss fights against Phantom Beasts: Ashura, Leviathan, Odin, and Bahamut. Only one of these was particularly difficult. The others required very specific strategies rather than a highly leveled party to defeat, making them closer to Puzzle Bosses than anything else. This was fitting, as the battles were intended to be tests of your skill. Furthermore, several powerful weapons and armor in the final dungeon are guarded by horribly powerful Palette Swap bosses, including one that hits the entire party with unblockable Doom when the fight starts (giving you a time limit before Total Party Kill).
 * The GBA version added some more, most notably, a modified version of Easytype Zeromus.
 * The DS remake removed most of those added by the GBA version, but features two new optional bosses only accessible through New Game+ - Geryon and Proto-Babil.
 * Final Fantasy VI Advance adds the bonus boss Kaiser Dragon, a monster that was Dummied Out of the original game and, due to having an associated monologue, seems to have been intended to have been a bonus boss in the original version, but to have been cut for time.
 * In addition to Kaiser Dragon, the entire Bonus Dungeon leading up to him is full of bonus bosses, in the form of souped-up versions of the Eight Dragons, each much stronger than other bosses in the game.
 * There are also three new boss espers - Leviathan, Gigantuar, and Gilgamesh.
 * Also, there are five bosses found wandering in a pit within the Dragon's Den - Dark Behemoth, Abyss Worm, Gargantua, Earth Eater, and the Malboro Menaces. All but the Malboros are relatively easy to take down, though. On the other hand, you have the three bosses-in-chests: Neslug, Plague, and the Flan Princesses, which use massive recovery, Doom to all, and Berserk to all, respectively.
 * Final Fantasy V had two of these, Omega and Shinryu, both who gave trinkets praising your deed upon death, and who made Neo Exdeath look like a marshmallow peep in comparison.
 * Final Fantasy V Advance was especially brutal with these, creating an entire Bonus Dungeon full of them. This included such prestigious opponents as Omega MK II and Neo Shinryu, souped-up versions of the bonus bosses of the original game, both of whom made their originals look like marshmallow peeps in comparison (Omega MK. II was very notable, housing a huge floor full of copies of Omega, each one as strong as the bonus boss of the same name, proving just how much stronger the players had to be to stand a chance.) But they are much weaker than the SNES originals.
 * Keep in mind that all of this being harder than the rest of the game is no mean feat - this was, after all, the Final Fantasy that was denied release outside of Japan first time around due to being too hard.
 * Ozma (unique in that his difficulty has little to do with inflated stats and almost everything to do with proper strategizing, albeit with more than a hint of Guide Dang It) and Hades in Final Fantasy IX. Kinda complementary - Hades turns out to be a legendary synthesist, and one of the rewards for beating Ozma is something you can synth off to obtain Ark, the ridiculously over-the-top summon.
 * There's also the Tantarian, another boss whose difficulty is based on strategy rather than just stats. Beating him nets an accessory that teaches the very useful Auto-Haste ability.
 * ...and Final Fantasy XIII continues the tradition with Vercingetorix, boss of the final Mission. This bad boy has 15.8 million hitpoints and can't even be accessed until after you beat the game. Long Guis probably also qualifies, having even more HP and huge stats.
 * Final Fantasy XI actually has a entire class of monsters for this: Notorious Monsters. While some are actually related to a quest or mission story, the vast majority of them only exist to kill for epik lootz and Loot Drama.
 * Final Fantasy XI undoubtedly has the most vicious on this list. One guild attempted to kill the newest boss, "Pandemonium Warden," but gave up after fighting it for eighteen straight hours. Then there's Absolute Virtue, which has never been beaten without the dev team completely removing the method for doing so after it's been found. Ever.
 * Final Fantasy XIII-2 features some optional bosses too but just three of them take the cake. There is Yomi a weaker version of Vercingetorix(but still formidable!). Long Gui can be challenged again and finally there is Raspatil who starts as a random encounter but if challenged, gives you a run for your money and is considered the hardest fight in the game.
 * The DCL Colosseum battles are all nasty, especially Jhil, the one person everyone who played XIII wanted to kill and now can't because of her summoning tricks and Sadistic Surge, and Gilgamesh, who has almost 10 million HP and after knocking off half of it will speed up and start spamming daze.
 * Kingdom Hearts loves these.
 * First game: The Clock Tower Phantom (gives you the last Stop spell upgrade), as well as many of the tournaments.
 * The international release added Sephiroth (bragging rights), Ice Titan (bragging rights), Kurt Zisa (bragging rights),
 * The Final Mix version adds in the international bosses, gives you decent rewards for beating them (extra Keyblades for Seph and Ice Titan, and the Zantetsuken attack for Kurt), and adds the Unknown, who is later revealed to be.
 * Kingdom Hearts II: Sephiroth + tournament matches again. The Final Mix included multiple upgraded versions of the Organization (including the ones who had died in the previous game), made an actual fight instead of a Cutscene Boss, and filled the "Unknown" slot with a mystery Keyblade Master named the "Lingering Sentiment" who is later revealed to be . He is found via a portal at Disney Castle and is THE hardest boss in the entire game.
 * 358/2 Days: Dustflier. He can be found before beating the game, in a late-game mission where the goal is to defeat six bosses scattered through Twilight Town in succession, and only appears after all six are dead. And while you can complete the mission by only taking out the first six, you have to go and finish Dustflier to fill up the extra portion of the Mission Gauge, which is part of Hundred-Percent Completion. (Fortunately, 358/2 allows you to redo missions at will.)
 * To give you an idea about that mission, it is implied that the Organization is actively trying to get Roxas killed on that mission. Those six bosses are HARD, but look like a joke next to Dustflier. EVERY SINGLE ONE of his attacks causes some sort of status effect. They hurt pretty bad, too. He spends most of the battle out of your reach, too. So no, the developers were NOT trying to be fair.
 * To elaborate even further, his attacks are: A powerful ground stomp that covers the whole field (avoidable only by timing your Glide correctly) and lasts much longer than it should, a very fast and almost impossible to read tackle that makes him invincible, a kick attack so strong that could as well be a One-Hit Kill and Meteor Breath, a homing multi-hit attack which, to put it simply, if you don't dodge, kills you. No, not even Auto Life will save you. And as said above, EVERY one of those can deal a random status effect.
 * Birth By Sleep brings its own Bonus Boss,, notable in that he only has one health bar because he's JUST THAT HARD TO HIT. If you heal yourself during the battle, he also heals himself. Completely. The Iron Imprisoners are a much less difficult example.
 * Birth By Sleep's international release adds a new boss, the imaginatively-named 'Mysterious Figure,', a black-coated, dual-laser-blade-wielding mofo clearly in the same style as the first Final Mix's just as imaginatively-named Unknown. This boss has put the rest of the series's famously batshit hard bosses to shame. So far, the only known strategies for beating it are "Spam Dodge Roll with Ventus and pray" and "Spam Thunder Surge with anyone and pray".
 * The Final Mix adds adding three more Bonus Bosses that can be fought in the Mirage Arena: Monstro and the armors of Master Eraqus ("Armor of the Master") and Master Xehanort ("No Heart").
 * Dream Drop Distance has Julius, featuring heavy defense and powerfully strong attacks.
 * Mewtwo in Pokémon Red and Blue, Red in the second generation (though it's debatable whether he's this or the True Final Boss, since the credits roll after you win), and as mentioned below, Steven in Emerald Version.
 * The Generation IV games, Pokémon Diamond and Pearl/Platinum, also give us Heatran, Giratina, Cresselia, and Regigigas; four legendaries that can only be encountered after you've beaten the main storyline and obtained the National Dex (with the exception of Giratina in Platinum).
 * Latios/Latias and Rayquaza in Pokémon Ruby and Sapphire/Emerald. Rayquaza can be captured before you take on the Elite Four in Emerald (and Groudon and Kyogre take its place as Bonus Bosses.
 * Steven turns from Final Boss in Ruby/Sapphire to Bonus Boss in Emerald, having stepped down from his position as Champion. You find him in a secluded cave in Meteor Falls with a team identical to the one he has in Ruby/Sapphire, except twenty levels higher.
 * In the first Generation II games, the legendary that's not your version mascot also counts as a Bonus Boss; in Crystal, both Ho-oh and Lugia also count, with Lugia being available only by talking to a man in Pewter City and Ho-oh only being available by catching the Legendary Beasts.
 * HeartGold and SoulSilver have a boss Trainer that can only be fought by having a Celebi: . This Trainer isn't very difficult, however, and mainly serves to tie up some loose story ends.
 * Those two games also have a bonus battle with Red, the player character from Pokémon Red and Blue. He is the toughest trainer in the games to date, with all of his Pokemon over level 80. You have to massively level grind after the main game or transfer Pokemon from other games to stand a chance against him.
 * Pokémon Black and White have as their bonus boss, as well as.
 * The use of cheats also ables the player to battle Professor Oak in the original Red/Blue versions, with a team of higher levels than the freaking Champion. Some fans speculate that he was a supposed to be a True Final Boss and got removed from the storyline, but the producers forgot to remove his battle data, since Boss Rush Agatha of Elite Four states that he is a Retired Badass. Sadly, he didn't get any battle data in FireRed/LeafGreen.
 * The various Frontier Brains of Hoenn and Sinnoh/Johto's Battle Frontiers, as well as the Subway Bosses of Unova's Battle Subway, can be considered this in the main series Pokémon games after one defeats the Champion.
 * All of the Legendary Pokémon let alone the Legendary Birds, Groudon, and Rayquaza from the first Pokémon Mystery Dungeon games, and most Legendary Pokémon that have no relation to the plot in the second set of Pokémon Mystery Dungeon games.
 * Kangaxx in Baldurs Gate Two, who could only be hit by + 4 weapons, and, in his demilich form, could cast the annoying insta-kill spell Imprisonment at will, which had the tendency to mess up romances if your love interest got hit. Kangaxx does have an Achilles' Heel however, in the form of a shockingly poor death save for his level. In the expansion pack Throne of Bhaal, the Bonus Boss was Demogorgon, an incredibly powerful demon imprisoned at the bottom of Watcher's Keep. Most of the dragons in the game are also optional fights, though they give some good rewards after being slain.
 * Divine Dragon/Divinegon in Dragon Quest III became available to fight after beating the game. You even needed to beat him five times to gain access to all of his wishes.
 * The Game Boy Color version of this game introduced yet another bonus boss, GranDragn. Getting to this boss required you to complete such a long, boring, and ridiculous fetch quest that even the most hardcore and dedicated gamers have never seen him.
 * And it's a completely Luck-Based Mission. Yeah.
 * God in Dragon Quest VII. Seriously.
 * And the four spirits, which is the odd thing since they are supposed to be weaker storyline wise then both the Last Boss and God, yet you can't fight them unless you can beat God easily.
 * The Dragon King in Dragon Quest VIII.
 * Have fun with the Darksteel Dragon. He's basically a Metal Slime with a lot better attack, who attacks three times, and, just for more fun, he has nearly 2000 HP.
 * Pretty much half of the bosses in Dragon Quest IX. Of special note are the grotto bosses,, and the legacy bosses, the final bosses and some midbosses from every previous serial title (e.g. Dragonlord, Zoma, Estark, Rhapthorne, etc.) The best kind of Pandering to the Base. You can even opt to give the legacy bosses the experience you gain from beating them, and they will level up each time, to a max of 99.
 * Dragon Quest VI has Nokturnus (better known as Dark Dream), who, thanks to his appearances in the Monsters series, is considered by many to be the quintessential Bonus Boss of the series.
 * Nokturnus actually gets a little more plot relevance than the average Bonus Boss, in that during the story, you witness a king try to summon him to deal with the Big Bad, only to be violently reminded that Evil Is Not a Toy. Though, if you're strong enough to put Nokturnus in his place, he really will deal with the Big Bad on your behalf.
 * In the SaGa series of games, most of the game is optional, including many of the bosses. Saga Frontier does have a few optional bosses who are particularly difficult, including the Earth Dragon in the Bio Research Lab, and cheating bastard Jotnar, who likes to employ his most powerful attack four consecutive times on his second turn.
 * Golden Sun had Deadbeard, an undead pirate found at the bottom of Crossbone Isle, who guards the game's most powerful armor. One path to him contained another bonus boss, a weather controlling lizard.
 * The Lost Age gave us the Star Magician, Sentinel, Valukar, and Dullahan. The Star Magician summons mooks to use Jupiter psynergy on you, buff and heal the Magician, and explode for huge damage. Sentinel constantly buffs his defense and is immune to all psynergy, meaning he gets tougher and tougher. Valukar can knock your Djinn into Standby and use your summons against you with Crucible. And Dullahan can put every active party member's Djinn into recovery with Djinn Storm, gets three moves per turn, and hits like a runaway cement mixer.
 * In Dark Dawn, Star Magician and Dullahan get buffed up, with Star Magician getting three new mook types (Curse, Death, and Ghoul) and Dullahan getting Valukar's Crucible move. There's also the Ogre Titans, a group of five increasingly powerful physical attackers, and the Ancient Demon, who can take over one character with Demon Sign.
 * And in the early parts of Dark Dawn, if you try to beat the Psynergy Training Grounds a second time, the Dim Dragon gets an upgrade, making it an early-game Bonus Boss.
 * Super Mario RPG has two: tiny martial arts master Jinx, who you must fight three times (after defeating his apprentice, Jagger), and Final Fantasy Shout-Out Culex, a powerful magic-using entity from another dimension who attacks using four elemental crystals and is quite possibly harder than the game's Final Boss. Both of these bosses live in Monstro Town.
 * There's also Mokura, a green cloud monster that appears randomly in Belome Temple and has boss music playing during his fight, although he's not nearly as tough as the other two.
 * Paper Mario has Bloopers, who show up every time you find a new shortcut in the sewer. Kent C. Koopa also blocks a road and makes you pay to pass; beating him lets you pass for free. Then there is a Dojo, where you can fight for bragging rights. The final blooper, Kent C., and the Dojo Master could all give Bowser a run for his money if he didn't have the Star Rod.
 * I And you don't really get anything for beating the Master, even experience, though you can't get a gameover on any dojo fights either. Defeating Kent C. gives no major reward beyond clearing a path he was blocking that you can just go around.
 * The Bloopers are a little odd in this regard - one of them is mandatory, but which one that is depends on how many you've fought. If you've already fought Blooper and Electroblooper by a certain point (which is likely), then you'll have to beat Super Blooper. If you haven't, you'll have to fight either Blooper or, if you've already beaten it, Electroblooper.
 * Bonetail in Paper Mario: The Thousand-Year Door is literally more powerful than the game's final boss (which is lampshaded by Goombella) and requires going through 90 (99 floors with a "breather" every 10) battles to reach him. There's also Atomic Boo, but that one's easier.
 * Bonetail may have higher stats than the final boss, but trust me, the final boss is a lot harder. She fully heals herself halfway through the fight by consuming your audience, can heal herself for small amounts by draining your HP, and has numerous nasty attacks. Eep.
 * Part of Bonetail's difficulty came from having the go through the Pit of Hundred Trials, getting through that is much harder then the fight with him.
 * Super Paper Mario doubled it, though.
 * Mario & Luigi: Bowser's Inside Story has several:
 * Tales (series)
 * Tales of Symphonia had at least 6 hidden bosses, the second of which gives you weapons that makes the rest of the game ridiculously easy - as if it weren't already for anyone powerful enough to defeat him in the first place.
 * Tales of Phantasia had Pluto, who gave you his summon, and Odin, who is also a Duel Boss. Apparently, he really was that Odin.
 * Only in the remake; the original had neither of these. It did have a long, hard Bonus Dungeon, though.
 * Tales of Eternia had Dhaos Sekundes, the summon of Time, although you didn't have to beat him to enlist his services. Maxwell and Shadow were also technically optional, but you did have to beat them. Cless gives you his bandanna and the Eternal Sword if you beat him in the Arena, and Valkyrie stands before a lot of treasure chests. Things like this make Eternia end up a very long and interesting game.
 * The true bonus boss of Tales of Eternia is Nereid Regulus, and he lives up to that reputation too.
 * Tales of Vesperia has the "Traitor to the Heavens" . It also has a bit of an odd case with the Radiant Winged One,
 * Vesperia also features as bosses in the 200 Man Melee.
 * The PlayStation 3 Updated Rerelease adds several more optional bosses, such as Clint of the Hunting Blades, the Sword Dancers, and.
 * There's also a colosseum fight where you use the four characters you currently have in your party against computer controlled versions of the other four you aren't using.
 * In addition to the already long list of bonus bosses in Vesperia, there's also Yeager's bodyguards, Gauche and Droite (aka, The Loli Twins).
 * Tales of the Abyss contains, who's practically impossible to beat on your first playthrough, as well as Reid, Mint, Nanaly, and Philia. (Reid is annoying... you think saving him for last is the good thing, right? AURORA WALL! Oh damn it, he just revived his harem!)
 * She gets a lot easier if you use hit and run tactics utilizing the Free Run mechanism. Still a lot harder than any other character in the game due to her raw power, but probably only as hard as a moderately hard boss fight would be in a pre-Free Run Tales game.
 * And she's arguably harder in the NA version. Yeah, she's tough in Japan, but just the mystic artes alone... ugh. In the Japanese version, you only had to worry about Big Bang. Oh, no problem - I'll just counter that with a treat or have Tear use Holy Song. She wasn't that bad in the Japanese version, so let's try the - wait a second, what's she doing using Indignation on me?!? She didn't have that before! What the- She has Fortune's Arc, too?! And Rending Sabre, Mystic Cage, and Innocent Shine? Oh but I'm not done yet... FRAGMENTED END. Ugh.
 * She has Time Stop too in the American version. Why is that a problem? She tends to either cast Meteor Storm while time is stopped or goes up close and uses Rending Saber right after time starts flowing again. At the very least, Big Bang, Mystic Cage, and Fortune's Arc can't kill everyone. But, that doesn't stop Fragmented End or Indignation from doing that, just pray that you aren't waiting near your healers to heal you when she uses them.
 * Tales of Graces has the post-game boss Solomos, as well as all the other bosses in the Zone Cage and Coliseum (including  and  . Defeating Solomos also changes the real final boss,   into  . In addition, you can re-fight your first boss, although he is no longer the Warmup Boss that he was before. You also can fight the incredibly large and strong Rockgagan, although in the PlayStation 3 version, you only really get a trophy for defeating him - not even experience points.
 * Also in Graces, you have the three dragons that correspond to the 3 Giant Cryas in each country. Good luck trying to take them on before beating the final boss cause even on Normal, its not easy.
 * The four-legged Dragons (not to be mistaken with the two-legged wyverns) in Monster Hunter, as well as Kirin the lightning unicorn.
 * In the Mega Man Battle Network series, there is so much extra content and so many bonus bosses that the time taken to defeat them is longer than the main story line. In all of them, however, you face Bass/Forte. Unlike other continuities, here he is a god-like Badass.
 * The sequel series, Mega Man Star Force is similar, although it doesn't take nearly as long to do so. Usually there's a secret area after beating the Final Boss where you have to fight upgraded forms of each boss in the game, followed by an all-new secret boss. After doing that, the storyline's final boss is upgraded, with it being the truly strongest boss in the game. The only exception to this is the second game, where after beating the upgraded final boss, you fight
 * You can also engage a number of repeatable bonus bosses and random people who can Wave Change in 2. These include second shots at the storyline bosses to get their Mega cards, plus farmable bosses like Kung Foo Kyd, Gemini Spark, and Cancer Bubble.
 * Mega Man X Command Mission, meanwhile, had two Bonus Bosses plus another set of nine. Rafflesian and Duckbill Mole gave X, Zero, and Axl new abilities that were useful in what remained of the game by that point. Ninetails and the preceding eight Tails Clan members, on the other hand, who were definitively even more this trope, could only be fought after beating the final boss, making the rewards for beating them pretty much awesomely worthless.
 * The 3 Golden Pigs at the end of the Bonus Dungeon (Mull's Dungeon) in Atelier Iris, which are significantly more difficult than the final boss.
 * In Atelier Iris 2: The Azoth of Destiny, the fights at the Dragon's Nest, particularly the last one against 3 Instant Brownies. However, while they are more powerful than the last boss, the overly-easy battle system makes them no more difficult than anything else, provided the player has stocked up on resurrection items.
 * Kisuke in Bleach: Soul Carnival 2. The thing with him, though, is that he can be fought pretty early on in the game, not that you'd have any chance of survival then. Until you clear the Soul Society missions, he'll probably kill you with just one combo. Not to mention that he has two supports, whereas most bosses only have one, OR the fact that his Burning Attack can even hit you if you're off-screen, and it stuns you invariably (as Burning Attacks never miss). When you defeat him, you'll unlock him as a playable character.
 * Breath of Fire III had the Berserker, difficult because it attacked for three to four hundred damage, and often attacked several times before any of your characters got to. Worse, it was a random encounter on the way to getting one of your dragon genes, and was a normal monster.
 * And in the same area was an even tougher Bonus Boss, the Arch Mage. He has far less HP than Berserker (only about 3,000 compared to the Berserker's 15,000), but regenerates 1,500 of it every single round, which is more than most casual players can deal in a single round, and is capable of hitting just has hard as Berserker. In fact, if you have one living and two dead characters, Arch Mage will USE A SKILL THAT REVIVES THE TWO CHARACTERS. Presumably, he does this simply as a means of embarrassing you further.
 * Breath of Fire IV has Rider as a hellish random encounter in Mukto. With 40,000 HP he is already a tough opponent but he also regenerates 20,000 HP every turn...try to overcome that.
 * Breath of Fire: Dragon Quarter had Dover, who was the final boss in the Bonus Dungeon. As long as you have a high enough level (around 50), he's not that bad. The only problem is is that his defense switches from regular ol' attacks to Dragon defense. So your time limit is based on how much you have left on the D-Counter. Lin's/Rin's "Shatter!" technique is particularly effective here.
 * The battle in Shin Megami Tensei game Digital Devil Saga, quite possibly one of the hardest bosses in the history of JRPGs, if not the hardest boss. Not only will he instantly kill you in the first turn if you have a "forbidden" move set, but the strategy required to defeat him requires a very specific use of one usually worthless skill that you would never use in the main game. I have no clue how anyone could figure this out without the use of a strategy guide. Beating him gets you a useful accessory that you can only use in the sequel.
 * And even then, if you do manage to get him down to half health, his High Pixie will fully heal him. If you kill all of his demon companions, he'll insta-kill you. If you go in with any negating moves (the said 'forbidden list'), he'll insta-kill you. Hell, even with the correct strategy, if you take too long to kill him, he'll just get bored and insta-kill you anyway. To add insult to injury, his battle music is the regular Nocturne battle music, so he may be an uber hard bonus fight to you, but you're nothing more than a random encounter to him.
 * in the sequel is likewise enormously difficult, if a lot less so than the . Anyway, even without their instant kill skills, both are beastly difficult -- you need to cap out your major stats to even have a fighting chance, and then there's the actual strategy involved... which simply fails half the time due to bad luck.
 * Shin Megami Tensei I and Raidou Kuzunoha vs. the Soulless Army has Beelzebub. Strange Journey has Alilat and Demiurge. Raidou Kuzunoha VS King Abaddon has Masakado. Atlus loves making these noble fellas to rain Megidolaons upon us.
 * The Shin Megami Tensei I example is notable for being one of the oldest Bonus Bosses, seeing as the game is from the early 90's. That's right: Atlus has always been doing this!
 * The Persona series:
 * Persona 3 has Elizabeth, although she can only be fought on the second playthrough after finishing all of her requests. Although she seems to be nothing more than a thin blonde girl dressed like a bellhop, she is the most powerful foe in the game, can only be fought one-on-one (she'll kill any other party members right away), gets to attack twice every turn (where you can only attack once), has multiple personas and powerful attacks, and heals herself completely when at low health. Even worse, if any of her attacks are nullified, reflected, or absorbed (which they invariably will be, considering her attack rate and huge range of attacks), she goes berserk and spams a 9999-damage attack (out of max possible HP of 999) nonstop until you die. Amusingly enough, the Updated Rerelease version of the game features an upgraded version of your starter Persona which seems to be tailor-made to fight her, as it resists every type of attack, giving you a good overall defense against anything she tosses out and avoiding driving her berserk.
 * While nowhere near as insane as Elizabeth, the Reaper that can be fought in both version of P3 is no slouch when it comes to making a normal player tear their hair out. Fond of casting spells that hit the whole party for 700-800 damage and going insane on unblockable "almighty" spells if the player sets up reflecting items. This one is arguably more infuriating to the average player due to the fact that he appears when the player spends to long in Tartarus, the game's central randomly generated dungeon.
 * Persona 3 Portable allows you to fight Elizabeth's brother Theodore if you make the proper choice early on in the game (it's not dependent on gender, apparently). However, it cranks things up to 11 with the Vision Quest, which not only contains more difficult versions of the Arcana Shadows, but also allows you to fight
 * Persona 4 continues the tradition with Margaret. While you can bring in party members and she won't (immediately) wipe them, she's just as frustrating. Nice changes include only healing once. She'll still 9999 Megidolaon you if you bring in a forbidden item, and dump lesser ones on you every 50 turns. She'll also exploit 1 More relentlessly if you give her the chance in the pattern.
 * After defeating a dungeon's main boss, a new, optional boss appears in the original's place. These tend to be a few orders of magnitude harder.
 * Persona 2 was the Persona series' first entry in this list with Philemon's brutal bonus battle on his EX Dungeon.
 * Devil Survivor brings in the Fallen Morning Star, Lucifer, for its battle. While insanely difficult (infinite range, level 99, etc.), beating him does give you the ability to fuse him. Most people just take advantage of the fact that Recarm gives the revived the next turn to attack, and just suicide run him.
 * Though your strategies are moot if he manages to get Megidoladyne off a couple of times, since every cast boosts its power until the 4th/5th is LETHAL. The first already does 500-600 on everyone!
 * Devil Survivor 2 has a whole slew of these, culminating in Alice. She isn't as difficult as Lucifer, but she does have Belial and Nebiros fighting with her. Belial doubles as a callback to the first Devil Survivor.
 * Tholapsyx, the red dragon from Neverwinter Nights 2. Quite possibly the hardest boss in the game, thanks to her size and fire attack; buffing the entire party (preferably with Energy Immunity: Fire and Stoneskin) and micromanaging spellbooks is almost mandatory, as opposed to the final bosses, whom you can just whack with sheer force if the party is well-equipped. The reward is 200,000 gold for your keep plus an insane amount of loot, including a cool weapon for paladins and clerics who bothered to take a certain quest in the keep.
 * The expansion pack has its own bonus boss, a seemingly harmless badger spirit that turns into a Gigantic Angry Badger of One-hit-kill Doom if you manage to anger it. Killing it nets one one of the only items in the game to grant permanent haste status (barring the time-consuming item crafting).
 * Etrian Odyssey series as a whole have a LOT of bonus bosses; many of them are usually part of SideQuests.
 * To start off, by the middle of the game, you can meet the first Bonus Boss, Wyvern, who is probably about 30-40 levels harder than your current, probably level 20s characters. Then you'll eventually meet three more bonus bosses, Golem, Alraune and Manticore; the former is a seriously extreme Mighty Glacier, the second loves to spam Standard Status Effects, and the last one combines both.
 * Most excruciating are the four post-game bonus bosses: The three dragons and Primevil. Each dragon has a breath weapon that can instantly wipe out your party on turn one. You can't beat them without having a protector with antivolt/antifire/anticold lvl.5, and Primevil has all three of those instant death attacks, so your protector has to have all three anti skills, and you need a turn guide so you know when to use which antimove. And you can still outright die if he uses his "disable all skills of all characters" move. You had to pray he didn't use it, or kill him very quickly. Good times, good times.
 * Its sequel, Heroes of Lagaard, also has a slew of bonus bosses. The dragons return, as well as a new True Final Boss and even more Sidequest bosses, some of them already accessible before you beat the game.
 * The next sequel, The Drowned City, doubled the number of bonus bosses from the previous games, both new and recurring, and this time, there are far more BonusBosses you can unlock via SideQuests before you beat the game even once. Cue the much-needed level grinding for the Bonus Dungeon.
 * Panthera Cantus from The World Ends With You is the toughest boss in the entire game; that bastard's tough even on Easy level! It has the highest attack power of all the Noise and two separate bodies; . Trying to avoid getting creamed by both at the same time makes the fight quite epic, if not frustrating.
 * In addition, there are "Boss Noise" on various days, blue Noise symbols that lead to fights against much stronger Noise than average. For Hundred-Percent Completion, you have to beat them all on Hard. Feel free to whimper.
 * And in addition to THAT, additional Boss Noise symbols appear on various days after beating the game. This is more of a convenience for those going for Hundred-Percent Completion, since you don't have to go through the entire chapter to fight the boss again, and can retry the fight if you didn't get what you were looking for.
 * Unfortunately, some of these bosses were timed fights, so on your first time through, you won if you managed to survive for 30 seconds. No such escape clause in the post-game, and many of these bosses are still intimidating, even having killed the final boss.
 * The Black Rabite from Seiken Densetsu 3. Evil in its cutest form!
 * Icewind Dale II has two of these, both within the same chapter. The first is a Players don't actually have to fight it, and can achieve their goal in a much easier way, but the difficulty of the battle alone makes it worth it for many players. The other boss is  Victory gives the player the Holy Avenger, arguably the best weapon in the game. Unfortunately, this pisses off quite a few people itself, as the weapon can only be wielded by a Paladin, meaning that there is no reward for anyone without a Paladin in their party.
 * The Gundam RPG MS Saga has Ultima Gundam and Omega Gundam, made from a mix of parts from G Gundam and Gundam Wing mecha respectively. Their names are obvious homages to Final Fantasy's perennial Bonus Boss pair, Ultima Weapon and Omega Weapon.
 * Live a Live features several bonus bosses. The caveman chapter has King Mammoth, who offers a decent reward in the King's Fang, as well as the randomly dropped "Soda." The ninja chapter has two, one which offers a weapon that you can also get at the end of the chapter if you don't kill anyone, and another that drops an item that can deal a decent amount of damage if used in battle. The final chapter has about five, four of which drop the most useful equipment in the game.
 * Parasite Eve had a few in the Chrysler Building. While most of the bosses are color swaps of the storyline bosses, the giant cockroach and giant bee were exclusive to the building.
 * Lost Odyssey has seven: Persona, King Kelolon, the Cave Worm, the Blue Dragon, the Holy Beast, Legendary Spirit Sorcerer Fu and The Immortal One in the Backyard.
 * Players with access to Xbox Live can get a new downloadable dungeon with an extra boss.
 * While it's part of the natural plot, Chrono Trigger has Lavos at the Ocean Palace. It's meant as a Hopeless Boss Fight, but there's still the potential to defeat it with enough skill and planning.
 * The Updated Rerelease adds in some more traditional ones, including
 * Grandia Xtreme, rather than a new and unique boss, gave you the chance to go back to the old dungeons after beating the final boss, and in one of them you can fight a super-powered level 200 version of a boss you already faced.
 * Monster Rancher Advance 2 features Ragnarok, a wandering, special Dragon who only appears after beating the final boss. He will only appear to fight you once a year, and only if you have a specific kind of monster on your farm. If you're not prepared to fight him on the week he comes to visit, too bad for you! His stats are extremely high (especially considering when your monster can first fight him), and he's tough.
 * In the dungeon before the Point of No Return in Legend of Dragoon, you are given the opportunity to fight the spirits of the three dragons you defeated. They drop some useful attack items, and they're also guarding chests containing powerful equipment.
 * There's also the four Dragoon ghosts in Velwebb, and not to mention Magician Faust at Flanvel Tower, who is the most powerful enemy in the game. Beating him does get you a great reward though in the Phantom Shield along with 30,000 gold. Here's everything you have to go through to get to him, which definitely puts him in this trope by the above definition: Throughout the game, there is a side quest to collect a total of fifty stardust scattered throughout the continent. Every tenth stardust, when shown to the proper character, will give you a different item. The final item is a mirror that is required in order to face Faust. Faust is an insanely powerful Wingly who was second-in-command to Melbhu Frahma, but ends up being a dozen times more dangerous, possibly due to having been alive and studying/training for the entire time his "boss" has been asleep. You then have to find the entrance to Flanvel Tower, following a winding maze of teleporters just to get to the tower. If you do not have the mirror in your possession the first time you see him, he WILL kill everyone in your party with one blow a piece, and you will be unable to touch him. As it turns out, this first Faust is merely a projected image. The real Faust is able to cast his magic through the image even while being twice as deep in the dungeon. And so, obviously, you must finish the maze.
 * Riviera: The Promised Land has Hades, the boss who only appear in the extra content section after you complete the game and obtained the Key to Hell from the Zombie Dragon in chapter 6. His Breakout does heavy damage, but he only attack once every 3 of your each character's turns (estimated), so if you keep healing yourself with elixer and attack him, he's a pushover. Of course, the final boss can has its HP brought down by 53% with Fanelia...
 * All the Geneforge games have an 'Expert' level dungeon with the toughest Bonus Boss in the game. Most of them generate repeated creations to add to their attack power, and traps that deal extra damage unless the player has the skill to disable them or at the least reduce their effect. The worst is the Titan ofGeneforge 4, which each time it was weakened to low health would shift to a new form, with a new set of attacks, defenses and vulnerabilities, requiring the player to have mastered a wide range of combat abilities. And of course, leaving the dungeon to rest also reset this Boss to it's original form and strength.
 * Bahamut Lagoon has special "side quests" - essentially single battles - available throughout the game. One of them, appropriately named Hard Dungeon, is only available in the last seven chapters and is far more difficult than the final boss.
 * All three episodes of Xenosaga have some:
 * Episode 1 has four: two mechas, Din Gareth and Jin Gareth, the sharpshooter Great Joe, and Mintia, an evil version of MOMO.
 * Episode 2 has a metric-buttload of optional bosses encountered in the game's many sidequests.
 * Episode 3 has two mechas: Omega Universitas, and Erde Kaiser Sigma. The latter is the only mecha in the game who's fought without the use of E.S.'s.
 * Mana Khemia Alchemists of Al Revis had 5 Bonus Bosses in total: four that were cameos from other Gust games, and the fifth is a Palette Swap of the True Final Boss. Each of them are fought individually, but later three of them can team up for a sixth battle, which can be very difficult if the player's not prepared. The only saving grace for that battle is their common vulnerability to a certain character's normal physical attack.
 * After beating Machinedramon and finishing the main story in Digimon World, you can continue the story and find a new level with high powered Mook Digimon. At the end is Machinedramon once again, only this time his health is maxed out at 9999 and his stats are also quite high. This is all for bragging rights.
 * While it's "only" a Mod, the Knights of the Old Republic Brotherhood of Shadow . You're pretty much stripped of all your gear, and  . Comparatively, the canonical Final Boss fight against Malak is nothing.
 * Mother 3 features several. First there is Lord Passion who carries Duster's Infinity Plus One Shoes, Lil Miss Marshmallow guards a decent weapon for that point of the game, and the King Statue is just there for experience and is ridiculously easy if you know what to do.
 * The Doppelganger in the .hack//G.U. games is optional, but is ridiculously difficult and gives some of the most powerful weapons in the games.
 * The Xbox 360 game Blue Dragon has several Dragons that don't need to be beaten, but give the player a useful accessory if they are. Genuine Bonus Bosses include the Gold Mecha Robo, the King Poo and the Golden Poo.
 * Dragon Age has several optional bosses among its many sidequests. The Revenants are powerful undead warriors that are managable on their own but get downright nasty with backup such as the ones that drop the Juggernaut equipment. Gaxkang the Unbound is an homage to Kangaxx from Baldur's Gate. shapeshifts into a High Dragon. An actual High Dragon (who could be beaten easily if you surrounded it with Traps before you aggro). And many more.
 * The expansion gives us the Queen of Blackmarsh, a lightning-breathing spectral dragon who is quite unambiguously even more powerful than the Final Boss. Also, the Golems of Amgarrak DLC has the Harvester, widely regarded as the toughest boss in the entire Origins saga.
 * Dragon Age II helpfully marks its bonus bosses with dedicated Achievements you get for offing them: a Varterral, a High Dragon, Xebenkeck the Desire Demon (who happens to be an old chum of Gaxkang from part one), and Hybris the Pride Demon. The last one is particularly bad news.
 * Naruto: Path Of The Ninja 2 has the Kumite Dojo, which has many bonus bosses.
 * Resonance of Fate pits you up against  at the end of Neverland. Unlike many examples on this page, they are generally considered pushovers, especially compared to what you fought to get to them.
 * The Spirit Engine 2 has a variant: the bonus boss, Urtat Underval, is fought roughly halfway through the game rather than at the end. Another variant is that you fight him twice;.
 * Dubloon features The Quartet of the Seas, a group of four bosses each found in one corner of the ocean. Alone, they aren't dangerous, but just wait until you fight them all four at once in the Pirate's Graveyard. They will kick your ass.
 * Sands of Destruction has plenty, including a few solo bosses involving Toppy, Kirie, and Agan.
 * Ys VI: The Ark of Napishtim has Majunun, a blowfish-like creature that will kill you in one hit the first time you can access it. To have a chance at all, you need to nearly max out your EXP, which takes many hours, as all the enemies only give 1 EXP on the highest few levels.
 * Fossil Fighters has a huge slew of bonus bosses, one of which opens up before the final boss, and the rest of which appear afterwards, in the Playable Epilogue.
 * The lone bonus boss available before the game's end is Petey, a dino battler who demands to fight a team of three specific dinosaurs. If you take the time to grind those three specific dinos, he's managable, but if you're anything less than maxed, be prepared to hurt from it.
 * But after the final boss? Both Saurhead and the BB Trio reappear with "marathon battle" bosses, where you have to beat three of their teams with no break in-between. They're tough, and your reward is  from waaay back at the game's halfway point. You can finally fight Dr. Diggins, though he offers up nothing but bragging rights (and EXP, if you're looking to grind). You can fight a samurai who's been in the hotel room next to yours for the entire game, whose most notable trait is that his Trainer rank is one beyond "Master." Oh, and if you're feeling lucky, punk, you can go back to   and challenge   And that isn't even all of them!
 * Tales of Graces has several of these
 * Fallout has had several of these across all the games in the series.
 * Fallout 3 Has five or six super mutant behemoths who you can hunt down. Admittedly, one is a Mini Boss over the course of the main story, but even then, you could just skip the entire section.
 * Fallout: New Vegas has the four (five with Old World Blues) legendary creatures, which are reskinned, upsized, and overpowered versions of their normal species. Admittedly, a sufficiently leveled character can make short work of most but the   from the main game, and   from old world blues can usually kill you in... Two hits. Almost always.
 * There's another Legendary Deathclaw in Lonesome Road, named Rawr. It's a Lightning Bruiser, even compared to the Alpha Male. Defeating him allows you to build one of the most powerful unarmed from his talons, Fist of Rawr.  There's also Gaius Magnus and Colonel Royez, who are only available
 * Fire EmblemRadiant Dawn has Levail, General Zelgius' apprentice. While Ike fights a Duel Boss battle with Zelgius, the rest of your team takes on Levail and his army. Defeating Zelgius is all that you have to do to beat the level, and the rest of your troops don't even have to move, let alone fight Levail (who as a top level Sentinel equipped with the Wishblade is one of the few genuine threats you'll meet in the last quarter of the game). Many choose to engage him though, out of the desire to kill a few more opponents and maybe get their hands on The Wishblade.
 * Levail's predecessor as wielder of the Wishblade, General Bryce of Daien, is another Optional Boss. Appearing in the last level of the game, Bryce stands in the centre of the map, astride the easiest path to the Final Boss, Mad King Ashnard. It is however, entirely possible to avoid fighting Bryce by taking another route, although most players don't think to do so.
 * Baten Kaitos Origins gives us a few. There's, who are fought to tie up loose ends; one of them allows access to the game's True Final Boss. There's also Arma Prototype M, a.k.a. The Wicked Gawd, who is the final boss of the Coliseum and is absurdly hard.
 * Dark Souls, with it's Metroidvania inspired world design, is practically made of this. Eight of the game's 22 bosses are entirely optional.
 * In Alpha Protocol, the Blood Knight Action Girl SIE will offer to team up with you during the mission where you first encounter her. You can agree to the alliance, or you can attack her (or you can agree, complete the mission, then attack her when you encounter her again at the end of the mission). Not only will she not die when you win, but your rep with her will increase.
 * In Alpha Protocol, the Blood Knight Action Girl SIE will offer to team up with you during the mission where you first encounter her. You can agree to the alliance, or you can attack her (or you can agree, complete the mission, then attack her when you encounter her again at the end of the mission). Not only will she not die when you win, but your rep with her will increase.

Shoot Em Ups

 * Gradius Gaiden has one, in a way. Normally, on Stage 8, you face six bosses, but play it on the second loop of the game to face a seventh: the Beam Spammin' Heaven's Gate.
 * Always present in Touhou games ever since Story of Eastern Wonderland. The fans would probably riot and destroy Tokyo if ZUN ever released a "proper" Touhou game without one.
 * Perfect Cherry Blossom had an extra Extra Stage with an extra Extra Boss. She's the one that made Gensokyo. Frightened yet?
 * Lampshaded in the 11th game, Subterranean Animism. When playing as the Marisa/Alice combo, there's a conversation along the lines of: "Why are we here again? We already beat the final boss." "It's the Extra Stage, just do it."
 * Finish Gokujou Parodius, and you get to fight your way through a "bonus stage".Who awaits at the end?A fire-spewing laser-shooting warmachine-deploying robotic penguin.It's as awesome as it sounds.
 * Armed Police Batrider has a crapton of Bonus Bosses, all of which appeared in the Mahou Daisakusen series and Battle Garegga. Said bosses include Bashinet, the Stage 1 boss of Mahou, and Black Heart, the Stage 5 boss of Garegga that, thanks to the stage edit feature, you can fight as early as Stage 2.
 * Do Don Pachi Dai-Fukkatsu has the first four bosses of Do Don Pachi as bonus midbosses, triggered by fulfilling certain conditions.
 * Hellsinker, as with Batrider and Garegga, has loads of secrets. Defeating the Scarlet Queen will result in a secret form of the boss that unleashes hell for 15 seconds, then disappears. The secret form can be easily triggered if the player has the game at a high rank, but it can also cause quite a surprise by randomly appearing under normal circumstances. If that happens, you're in for a world of hurt.

Simulation Games

 * The Trauma Center series has the X operations, which have you performing simulated/imagined/secret/God-knows-what operations involving Nintendo Hard variations of GUILT, Stigma, or Neo-GUILT.
 * Lots of missions in the Ace Combat games feature enemy aces whose defeat is not necessary for mission completion. Unlike traditional examples of the trope they're not really harder than the compulsory aces. On the other hand, there is a more traditional one in, who would be encountered in an Ace difficulty run of the Gauntlet if you did well enough. Similarly are   in a certain Skies of Deception mission.
 * Two of them are available in the combat sequences-filled Dating Sim Mitsumete Knight :  and   of the Eight Generals of Valpha-Valaharian, the main enemy squad of the game. While not a storyline-related boss unlike the other two, Sparkster of the Rocket Knight Adventures series count too.

Sports Games

 * International Super Star Soccer Deluxe for the SNES introduces a bonus match against an All-Star team with perfect stats after you win the World Series. Not only is this team supremely talented, but all your players are tired or very tired.

Stealth Games

 * Metal Gear Solid: Peace Walker has the

Survival Horror

 * Most of the bosses in Dead Rising are Bonus Bosses. And the handful you do have to fight to complete the game are all That One Boss. The game is hard.
 * Technically you don't have to do any of the bosses. To finish the game you just need to be alive when the time runs out. You only need to do the quests if you want Ending A.
 * Fatal Frame has a particular optional boss only on the Xbox version. A samurai ghost named "Armoured Warrior" who only appears on the final night on the hardest difficulty, who can instantly kill you if he touches you AT ALL. Oh, and he's fully invisible too.
 * And he can't be pushed back with a critical shot like every other ghost can be so he's ALWAYS coming towards you without stopping. Again, being invisible, you can't see him but rather HEAR him, unless you get a shot of him fading in and out quickly while looking through the camera, which is the only real way to defeat him.

Third Person Shooter

 * Lost Planet: Extreme Condition features a Bonus Boss, but it's rather unusual. Relatively early in the game, you encounter a giant worm Akrid that can CONSUME YOU and takes a massive amount of damage before falling. You're expected and encouraged to run from it... but if you want the challenge, you can fight and beat it, effectively making it a Bonus Boss. There's even an Achievement for doing so. Later on, you can fight a giant Akrid moth.

Turn Based Strategy

 * Demon Supreme Overlord Baal in Nippon Ichi's Marl Kingdom series, La Pucelle Tactics, Disgaea, Phantom Brave, Makai Kingdom, and Disgaea 2. In fact, most Nippon Ichi games let you fight characters from their other games as bonus bosses, and they're always at obscene levels. Laharl, Etna, and Flonne from Disgaea are all Bonus Bosses in Phantom Brave, for example. In many cases, defeating them will recruit them into your party. While the final bosses of the games tend to be at level 90-100, the bonus ones usually start somewhere around level 1000. That's start mind you. In "Etna Mode" of the new Disgaea PSP remake, Baal is literally level 9999.
 * Mind you, that's not all that impressive. He's actually the third level 9999 boss you've fought by then. He does, though, have about 3 or 4 times the stats of the last one, on an area that randomly clones one character a turn (friend or foe, but clones are all foes), triples all enemy stats, stops all special abilities, and prevents lifting.
 * Special mention should go to his Disgaea 3 DLC. Max level, three copies of his Tyrant form (as shown in the page image), well over 400 million HP, and an evility that nulls out damage each round based on how many copies of himself there are on the field. If you don't act quickly enough, the enemy base panels in the back will summon more Baal copies. If you're still too slow, they'll start bringing out Omega Sentinels for the Baal clones to Magichange with. Either you finish it quickly, or otherwise you're screwed.
 * The re-release of Disgaea 2: Cursed Memories brought back Mythology Gag Pringer X as a Bonus Boss, and he's more dangerous than Baal up there for a good reason: he can become immune to any special that's already been used against him, meaning you can't spam the same attack over and over, you HAVE to round out your move set. Later on, you get the chance to fight Eight of them, and they will all become immune to any special used against one of them, but that's not all: You can later pass a bill that turns all of the Land of Carnage monsters from Uber to Uber Lord, making them not only stronger, but you can't capture them and the bill sticks for that cycle. If you go back to take on Pringer X's army again, their stats are maxed out at 40 million, meaning damaging them is a daunting task in and of itself, much less hitting them.
 * They return in Disgaea 4: A Promise Unforgotten as the final bonus boss.
 * In most Super Robot Wars games, there is a final mission and boss only available if you fulfil certain conditions, mostly either through finishing the game until this point within a certain amount of turns, or reaching this point while in hard mode.
 * Though all of them fit True Final Boss much better. The two most memorable for this trooper are at the end of Alpha Gaiden also being That One Boss, and Impact does one of the most shocking especailly if you played Original Generation 2
 * Rarely, there will be bonus bosses that can be fought in earlier stages, as well. These are usually powered-up versions of the boss you would normally fight, or an extra boss you normally wouldn't fight. Possibly the most notable example is that, after fulfilling certain conditions in Super Robot Wars Alpha 2, after defeating, he will come right back in the even more powerful.
 * In addition, Super Robot Wars: Original Generation 2, one of the early missions tasked you with escaping from three bosses that you aren't expected to beat until the end game with a full party. Naturally, you only have three medium-level units and a battleship to fight these near-final bosses with. However, if you stick around and manage to beat them, their leader breaks the fourth wall to praise the player, and then after a minute gives up a whole load of special weapons and items to "shut you up and make you grateful".
 * Luminous Arc 2 has the reoccuring boss for the optional That One Sidequest Spa Battle series, . Beating this multiple of times with New Game Pluses is required for the Hundred-Percent Completion.
 * Final Fantasy Tactics gave us Elidibs and the Zodiac summon.
 * While not really a "Boss" per say, the original Advance Wars has a bonus Mission after the Final Battle with Sturm called Rivals, where Eagle challenges Andy to one last go around. In order to access it, you have to play through the Campaign and choose Sami for every Green Earth Mission (barring Eagle's introduction early on of course). This is noted to be quite difficult, as Sami's Green Earth Missions are easily the hardest of them (it doesn't help that she's not nearly the terror that she is in later games either), but it pays off in more ways than one; not only does it give access to Rivals, it also places Eagle as the 3rd CO in the Final Battle, along with an entire squadron of air units to start off (considering the closest airport is neutral and well near the middle, this is a HUGE plus). On the flip side, if you play Rivals in the already painful Advanced Campaign, you're basically doomed to be playing the map for a long time.