Dirtbag is a mole who was mutated by Bebop and Rocksteady accidentally during a trip to the zoo.
Groundchuck was created during the same zoo accident as his best friend dirtbag. It goes without saying that whatever zoo has a bull and mole exhibit is clearly not taking their job seriously.
Baxter was already fairly evil before he turned into a fly in a teleportation accident (a common occurrence in the '80s). He was pretty insane, but he did invent the MOUSERS, which are cool, and his van was turned into the turtle van.
Leatherhead was just a common sewer alligator until he swam through some ooze and turned into a Cajun survivalist. He likes to make guarantees.
StargazersThe RadicalsCosmic ForceThe Neutrinos
The Neutrinos are "cool teenagers" from Dimension X. Although they seem pretty lame and desparate to us Earth viewers, they're very hip and trendy in alternate dimensions, apparently.
Rat KingSewer RatSewer LordRatsputin
The Rat King
The Rat King lived in the sewers and could communicate telepathically with rats. He was basically a decent man, provided you accept the premised that humans should be governed by rats and made subservient to their needs.
Bugsy MaloneLondon DryPinky McFingersShorty McCoy
Pinky McFingers is a mafioso who once kidnapped Raphael and a bunch of comedians in the hopes that their comedy powers could paralyze the city with amplified "joke waves." He is not known for his good plans.
Muckman and Joe EyeballThe Trash CollectorsTrashley and StalksStinker and Carl
Muckman and Joe Eyeball
Originally Garson Greer and Joe Junkee, these two innocent garbagemen were turned into horrifying trash mutants by Bebop and Rocksteady.
Oroku NagiHamato YoshiOroku SakiUsagi Yojimbo
Usagi Yojimbo is a rabbit warrior from another dimension where animals are in charge. He kicks a lot of ass for a rabbit.
Metalhead is a robot turtle created by Krang but later reprogrammed by Donatello to clean his room, which feels exploitative, frankly.
Bebop and RocksteadyTerrorpin and WargTokka and RahzarSlash and Dash
Tokka and Rahzar
Tokka and Rahzar are a snapping turtle and wolf mutated by Bebop and Rocksteady and later captured by Dirk Savage: Mutant Hunter. At one point, during their captivity, Rahzar says to Tokka, "You are my only friend," which is just incredibly poignant.
SlashDimension X DonatelloCrasherTerrorpin
Slash was Bebop's pet turtle, which is just a disaster waiting to happen. After a few scuffles with the turtles and an encounter with some aliens, he becomes a genius with a British accent (don't ask) and decides to turn everyone on Earth into turtles.
Punk frogsStreet frogsParty frogsRonin frogs
The Punk Frogs – Attila, Genghis, Napoleon, and Rasputin – were named by their creator, Shredder, after his personal heroes. They later become friends with the turtles, because they are basically pretty chill dudes.
Batwing and FlyboySkeeter and HardwareWingnut and ScrewlooseDingbat and Buzzsaw
Wingnut and Screwloose
Wingnut is a bat, and Screwloose is a mosquito, and they're from the planet Flagenon. They're not very nice at all, and for some reason, someone puts them in charge of a children's school on Earth. Havoc ensues.
Mutagen ManSly SludgeDr. OozeSkumm
Originally a deliveryman named Seymour Gutts, Mutagen Man gets his unique style from an accident involving a vat of mutagen. For some reason, he decides to be evil.