Breakout Mook Character
A spin-off game or story (or a side story or special episode) that focuses on a Mascot Mook whose only purpose was to die by the dozen in the original.
Usually will center around an exceptional member of the Henchmen Race. If the events of this story are canon this guy might become the Ascended Extra on par with the original heroes. Some of them will be hunting their own kind on the heroes' side, others will be trying to make it through Mook Promotion and become a Big Bad on their own right.
Usually played for comedy (it's funny to see things from the point of view of someone you didn't even consider important enough to be having any sort of private life or backstory).
- From the Star Wars Expanded Universe, the Zahn novel Allegiance.
- (above) Lime, the playable Slime character from Lunia. Basically during the entire first chapter he is technically slaughtering his own kind.
- The Minions in Overlord got their own spin-off on DS.
- Many, many Super Mario World ROM hacks.
- Rob-ombs Quest has a Bob-omb as the main character.
- The Ninji Safa has a Ninji as the main character.
- Pokey's Adventure has a Pokey as the main character.
- Super Blooper has a Blooper as the main character.
- Two upcoming hacks have a Buzzy Beetle and a Muncher as main characters respectively.
- There are two Mario flash games with a Bullet Bill as the main character, and another with a Bandit as the main character.
- And two where you are a koopa: 
- Dragon Quest Heroes: Rocket Slime stars a slime named Rocket who protects his band of expendable Slimes from other expendable minions.
- Mega Man:
- Similar to the Super Mario World ROM hacks, there was a "Hard Hat" trilogy of fan games where the player was a Mettaur.
- And also "Mega Man: A Day in the Limelight", where you could play as the robot masters from 1.
- Also there is Star Man and it`s remake Star Man Upgraded.
- Prinny, a Gaiden Game to the Disgaea series starring an armada of the series' easily combustible penguin mooks. This being Prinny, his role is still to die by the dozen: players thousand of lives are all separate Prinnies, who pass the Scarf Of Concentrated Awesome to the next in line. The sequel also makes Asagi (a recurring character who has a running gag of never being the main character of the game) playable as a Prinny (as she'd died and been reincarnated in the previous game).
- Paper Mario 1 & 2 might count, since Goombario / Goombella and other party members are technically main characters that are members of mook races.
- Star Wars:
- Ghosts N Goblins has a three-game subseries, Gargoyle's Quest, based on its iconic Red Arremers. Granted, they don't get picked off by the dozen, so they might not qualify.
- Left 4 Dead: Versus Mode (in all its incarnations) lets you control the Special Infected. Which one you get is determined randomly, but Tanks only appear at predetermined points along the map (and are awarded to the Infected player who's done the most damage) and the Witch isn't on the list at all unless you're using a custom campaign mutation.
- Half of Halo 2 consisted of playing as an Elite who had been promoted to Arbiter.
- Abobo from Double Dragon becomes the protagonist of the Flash game Abobos Big Adventure, where he gets to kick the asses of every other NES character ever.
- Lander/Randar which has made several cameo appearances in Compile games, got an MSX game Randar no Bouken.
- The spin-offs Opposing Force and Blue Shift featured a mook enemy and an interchangeable ally NPC as characters. Neither ever meet Freeman, but they're no different from the folks he kills by the dozen or completely neglects to rescue.
- The same for mods like Azure Sheep switching between male and female security guards and Point of View featuring a variant "Alien Slave" (Vortigaunt) as the Player Character.
- And a Half Life 2 mod known as "Human Error" puts you in the boots of a Civil Protection officer.
- Shin Megami Tensei regular Jack Frost and his friends Pyro Jack and Jack Ripper all star in their own spinoff, Jack Bros..
- One of the characters in Mana Khemia 2 is a little girl who was raised by three Punis, slime creatures that are common enemies in the Atelier. She constantly rides one of them and uses all three of them in battle.
- There is a game actually called Mega Man Day in The Limelight where you play as the robot masters from the first game.
- The somewhat obscure Namco game Grobda puts the player in control of a tank enemy from Xevious.
- Battle for Wesnoth has a lot of these, including "Inky's Quest"—The Cuttlefish Campaign. Inky recruits cuttlefish, tentacles, pearl-shooting giant clams...
- Variation: The "You Are A Creeper" Mod for Minecraft. In which, yes, you are a creeper.
- The sequel to the C64 classic Attack Of The Mutant Camels was called Revenge Of The Mutant Camels. Take a guess.
- In the DLC for Dungeons of Dredmor, The Realm Of The Diggle Gods, there is a skill that allows the Player Character to transform into a Diggle, the Mascot Mook of the game. Whether you pick this or not, you can still pray to the eponymous Diggle Gods when you get down to the lowest floors on the dungeon. Apparently the little targets can be pretty powerful when they're not under the control of the AI.
- Quest for Yrolg has a little imp killing off the Fighter, Mage, Thief heroes, in reversal of the Quest for Glory games.
- The sample game that came with the first Playstation RPG Maker tool has this as it's premise - Gobli was a goblin that dreamed of becoming a main character and not just XP fodder for the heroes (long before the webcomic with the same premise, mind). Joining him on his quest were a literal hole in the wall that wanted to be a character, and an adorable kitten that wanted to become a Final Boss.
- The Nitrome game Enemy 585 is about helping a Koopa analogue work his way backward through an Affectionate Parody of Super Mario World.
- If you buy Episodes 1 and 2 of Sonic the Hedgehog 4, you will unlock Episode Metal, in which you play as Metal Sonic through Episode 1's settings.