My guess is he's a rat.
If he was a mutated human he'd stay fairly human-sized, or at least be more "animalistic human" rather than "animal doing human things" (there is a difference between the two).
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"We're ninjas." Leo, the Samurai
"We're mutants!" Raph, the Outsider
"Technically, we're turtles." Donnie, the Nerd
"And we're teenagers." Mikey, the Teenager
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