Monster Allies

Basically where monsters in RPGs can fight alongside the heroes. Allied monsters' individual personalities may exist (Paper Mario) or not (Dragon Quest series to an extent). It's more likely in the latter case that defeating them first is a requirement for them allying with you—though, it's not unheard of in the former case.

There is usually a Slime/Goomba type monster ally that can/will join earlier than other monster allies; they're quite likely to be a Lethal Joke Character.

A subtrope of Mons. Please note that this is the case where the heroes always have HP whether or not the Monster Allies do, and frequently enough, the hero(es) is/are (a) designated survivor(s). A nonsentient Monster Ally will usually be a Pet Monstrosity. Monsters who are in fact the main characters are Monster Adventurers.

First-Person Shooter

 * In Half Life 2 you can temporarily befriend and get help in storming an enemy facility from the previously hostile Antlions with the aid of some special pheromones.
 * In a brief moment in Halo 3,.

Hack and Slash

 * In Diablo II, Necromancer class can create skeletons out of fallen enemies to fight alongside him. Until he learns the Revive spell, which is the pinnacle of necromancy because it animates a creature in a way that gives it access to all the intelligence it had in life but gives control to the Necromancer, they all turn into humanoid, human-sized skeletons—even if they were previously foot-tall imp demons. Or a cloud of flies. Or a ghost, or a bug. At least the imps might have bones.
 * This is apparently justified by Necromancers not actually using the bones of a defeated enemy for creating the skeleton, but rather just using the dead energy of it to animate other bones. Somehow.

Maze Game

 * The monsters in Sabre Wulf don't quite fit this, but go here better than under Mons—they're treated as tools that happen to be living (e.g. a large sleeping creature that functions as a trampoline, or a Chinese dragon that can be used as a platform.) Learning to command them takes many years of training, so the main character is the only person in the game who's shown to use them. Unusually for this trope, however, a clear distinction is made between "good creatures" used exclusively by you and "bad creatures" that accompany the titular beast.
 * Note that the above description doesn't match the game to which the wick goes, which is the one most familiar to 8-bit-era gamers; the ZX Spectrum version. Presumably Rare either remade the game, or at any rate recycled the title.

MMORPGs

 * The entire Hunter class from World of Warcraft revolves around this, though they can only tame one class of monsters, beasts. Warlocks do a similiar thing with demons except they have less options and only command a total of five demon types permanently, though they can also enslave most other demons for a limited time and can summon two more powerful demons that may escape their control.
 * Video Game/Mabinogi has several ways to get Monster Allies. The simplest is through the Pets premium-purchase feature. Pets have a limited daily summon time; and are subject to simplified versions of the aging, levelling, and skill mechanics. Pets can also be used as player characters, within the daily summon time limit; but can only interact with animal NPCs (two as of this edit). In the G3 expansion, the Dark Knight was released with the "Control of Chaos" skill; which allows limited, temporary control over any monster, with some skill-level based restrictions. As of G7, the Pet Taming skill allows all players to gain a similar ability.
 * Shin Megami Tensei Imagine. It's a series staple.
 * Lime the slime in Lunia, an unlockable Player Character.

Platform Game

 * Kirby Super Star had the option to turn copy abilities into allied partners.

Puzzle Game

 * Puzzle Quest: Some companions fall under this, and you can capture some monsters to use as mounts.

Real Time Strategy

 * Lost Magic
 * They're the basic units in Final Fantasy XII: Revenant Wings.

Role Playing Game

 * Dragon Quest V and Dragon Quest VI
 * Paper Mario and its sequel, arguably. The line defining "monster" in those games are quite blurred. Many of the partners that join Mario belong to the classic enemy types like Goombas and Koopas, but there exist both friendly and enemies of these. In some cases enemies don't even exist in that particular game. (Cheep-Cheeps and Boos in the first, for instance)
 * While all your allies in the first game are based on classic Mario enemies and have attacks along the same theme, the second game mixes it up with a few characters of species previously seen as non-hostile NPC characters, never seen before at all, or.
 * The Lufia games. Lufia II has "capsule monsters", a small group of monsters—each with their own element—that can be found outside of combat and recruited. Ruins of Lore allows you to capture and train any regular monster and teach them skills from other captured monsters. In both games, the player does not control the monster's actions.
 * Tales of Symphonia: Dawn of the New World allows you to tame and use up to two monsters in your party lineup.
 * Narikiri Dungeon X expands on the concept further, by allowing you to tame practically any enemy (Final boss included), allows you replace all of your standard party members with them, and lets you assume direct control of them, as well.
 * Kinda subverted in Arcanum: Of Steamworks and Magick Obscura - characters with a high enough Persuasion skill can get "Gar, World's Smartest Orc" - who turns out to be an upper-class human who takes after a distant, presumably orcish ancestor.
 * Taken to its logical end in Planescape: Torment by making everyone who can join your party a monster (or at least, non PC race) of some sort.
 * Except Ignus, who's... off for other reasons.
 * Three Words: Shin Megami Tensei
 * Mega Man Battle Network 3 allowed you to tame viruses and allow you to get a virus chip, which summons said virus
 * Infinite Undiscovery had Gustav, the pet bear of the Rebellious Princess (who happened to look like every other bear enemy, but red) while several Suikoden games allowed you to recruit intelligent monsters as allies (sometimes as part of the 108, sometimes not). In both these cases, the powerful monster PC takes up two character slots because of its size and strength.
 * In SaGa 2, you can compose your entire party of monsters, though in practice it makes for a much more difficult experience.
 * Geneforge series has as one of it's premises the techno/magickal art of Shaping, so the player, depending on class, can create at least one to use as a decoy, or surround themselves with a horde of them. Upgrading skills gets you more, bigger, monsters and more variety. In most of the games, your only permanent party members are your creations. They don't have their own personality, but you can name them and treat them as pets, or just use them disposably.
 * In some of the games, at least one path will allow you to gain a rogue Creation ally if your diplomacy skill is high enough. They have personalities of their own, and you may or may not have to defeat them in battle.
 * In Legend of Mana it's possible to obtain just about any non-Boss monster as a pet through one of two methods: either finding a Monster Egg, which will hatch into a random monster of that type, or by "recruiting" a Demi-Human monster in a certain zone (this requires having a certain mana level in that zone).
 * Oblivion included this with both the Frostcrag Spire DLC and the Shivering Isles addon. The Spire gives you the ability to create Atronachs (SIC) who would serve as your familiar until either dismissed or killed, and Shivering Isles allies you with non-hostile Gnarls near the end of the main quest. You can also get Golden Saint or Dark Seducers escorts once you've finished Shivering Isles, though you can't take them into anywhere that isn't a part of the Isles.

Tabletop Games

 * Second Edition Advanced Dungeons and Dragons had an entire sourcebook devoted to creating monstrous characters, titled 'The Complete Book Of Humanoids'. Third Edition had a similar sourcebook named 'Savage Species'. 3.5 brought a lot of that into the Monster Manual, and Fourth Edition put it all into the back of the Monster Manual.

Turn-Based Strategy

 * All of the Disgaea games let you make your own party of characters, many of which are monsters. In order to unlock a monster, you have to have defeated it in battle at least once, sometimes many times (because the cost of making monster units decreases with every member of the species you kill, and some monster classes are initially absurdly expensive) or throw them onto a base panel to capture them (only valid for non boss monsters of a decently lower level than your non-deployed units or the main character, depending on the game). They come with their own set of weapons to use, and can wear the same armor as humanoid characters.
 * This is a feature of most Nippon Ichi strategy RPGs, from La Pucelle on. In La Pucelle, enemy monsters can be directly converted into allies. In later titles, monster units are usually created from scratch by the player.
 * Final Fantasy Tactics:
 * Boco, Wiegraf's chocobo, which you can rescue and let join your party.
 * The Mediator class with the Invite ability, which works on monster or human foes. They also get the 'tame' ability that automatically invites any monster brought to critical due to their attack.
 * Reis -in human shape- whose main ability is to tame dragons, but can also tame other monsters by punching.
 * Byblos, the Apanda-class demon that joins as a Guest in the final floor of the Deep Dungeon, and joins permanently if it survives.
 * Final Fantasy Tactics Advance lets you tame monsters, but they can't join you in battle directly. Instead, the Morpher class can use their abilities.
 * Final Fantasy Tactics A2 has some monsters join you, but only as guests.
 * Chocobo Knights can also tame the birds, but they cannot exist separate from their tamer of they run off.
 * Ogre Battle and Tactics Ogre
 * Arc the Lad II