Squirrelanoids are mutated squirrels that have near-skeletal appearances and second mouths in their main mouth. They are able to reproduce by entering into a host's mouth, and split into two perfectly similar new Squirrelanoids, whereafter the host coughs them up.
During The Mutation Situation, mutagen canisters were spread all over the streets of New York, and the mutagen of one such broken canister was shown to be licked up by a squirrel, which later, in Invasion of the Squirrelanoids, mutated. The Turtles witnessed the first squirrel mutant hopping into the mouth of an old man, whereafter the Turtles bring the man down to their sewer lair in order to research the nature of the squirrel mutant and to protect other New York citizens from the dangerous creature. However, inside the man's stomach the mutant performed a cell division, splitting into two beings instead of one, and the man finally coughs up these two new squirrel mutants, that the Turtles then try to catch, but find it extremely difficult as they ran uncontrolled around their entire lair. After a while, one of the two squirrel mutants enter Raphael's mouth, starting the same process that was shown earlier. Raphael finally coughs up two new squirrel mutants, so that there are three in all. The creatures suddenly start glowing, and soon evolve into a stronger, final and true form (Obviously heavily inspired by the xenomorph-aliens). It was here, that Michelangelo named them the "Squirrelanoids".
The Squirrelanoids proved to be very tough opponents and put up a good fight, until Mikey lures them into a septic tank drain. They do try to get back out of the lowering water, but fail...
However, one is later seen ominously holding a sewer worker, foreshadowing their return in a later episode.
Squirrelanoids return in episode Fungus Humungous, as part of Mikey's hallucinations.
Their appearance bears a great resemblance to the Xenomorphs from the Alien films (right down to the smaller extendable mouth), but with a red skin color, and features like those of squirrels, including large fluffy tails, pointy ears and the identifiable two sharp incisors that proved to be sharp enough to bite off Metalhead's head.
Their Xenomorph-inspired design could possibly be a reference to the Pizza Monsters from the 1987 Teenage Mutant Ninja Turtles episode "The Case of the Killer Pizzas", also seen in the video game "Turtles in Time".