Squirrelanoids are a group of 9 ft. humanoid mutated eastern gray squirrels that have red near-skeletal-like appearances, glowing green eyes and retractable tongues with second mouths in their main mouth (similar to the Xenomorph from the Alien franchise). In thier original form, they look like ordinary North American gray squirrels, but when they are excited or angry, they show an unnaturally aggressive behavior and a long, movable gripper tongue, which they can use as an additional limb.
Rather than multiply like normal grey squirrels through ordinary mating, Squirrelanoids use a quasi-parasitic form of reproduction. To this end they are able to reproduce by entering into a potential host's mouth penetrating into the gastrointestinal tract, where they divide up as a single-celled organism and split usually into two perfectly similar new Squirrelanoids per reproduction. Both offspring grow up within a few hours to be fully grown animals, whereafter the host coughs them up.
As a Squirrelanoid spread to breeding in the mutant Raphael, a strange further transformation was observed, which radically impacted the appearance in the subsequent generation of Squirrelanoids. The Squirrelanoids produced in this litter were huge, monstrous versions of their kind. The reasons for this reaction are unknown, however, it can be assumed that Raphael's already mutated DNA must have had a decisive influence on the development of the embryos. One of the most striking changes is the transformation of the whips tongue of the first version to a kind of second head, which has its own biting tools and it's own throat.
During The Mutation Situation, mutagen canisters were spread all over the streets of New York that fell from the Kraang Stealth Ship during the Kraang's fight with the turtles, and the mutagen of one such broken canister was shown to be licked up by an earthly eastern gray squirrel, which later, in Invasion of the Squirrelanoids, mutated. The Turtles witnessed the first squirrel mutant hopping into the mouth of an old Homeless Guy, whereafter the Turtles bring the man down to the lair in order to research the nature of the squirrel mutant, and to also protect other New York citizens from the dangerous creature. It was discovered that inside the man's stomach through X-ray, 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 uncontrollably 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 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". When the Turtles are being overwhelmed by the Squirrelanoids, Splinter arrives and fends them off, allowing them to escape into the tunnels. The Turtles end up going after them to keep them from reaching the surface and causing havoc.
The Squirrelanoids proved to be very tough opponents and put up a good fight, until using some popcorn and his knowledge of comic books, Mikey baits the Squirrelanoids into a drain pipe and flushes them into the septic tanks drain until Donnie can come up with a Retro-Mutagen to use on them. They try to get back out of the lowering water, but fail...
However, one of the Squirrelanoids manages to escape and is later seen ominously holding a Sewer Worker who finds the bag of popcorn, foreshadowing their return in a later episode. However, that same Sewer Worker was later seen running past Casey and April, terrifyingly screaming about giant mutant squirrels at the beginning of Fungus Humungus. Which, the Squirrelanoid, also made a return appearance in. Though as part of Mikey's hallucinations as Mikey's worst fear after he was sprayed by one of the mushroom's spawns, and resulted of seeing Raph as on.
Though an hallucination, the Squirrelanoid itself was captured by the Kraang And it, along with two more, were detained with the other mutants in Metalhead Rewired captured by Kraang in order for them to control them. One is first seen with a Parasitica Wasp, Kirby Bat and Cockroach Terminator after they discover Metalhead opening their cells freeing the captive mutants, a Squirrelanoid was seen peaking out of its cell. The second one is seen with Dr. Rockwell and the Newtralizer when Slash punched one of the Kraangdroids, a second Squirrelanoid was seen in a background just above his arm. Another Squirrelanoid (seen with a Parasitica Wasp and Pete) helps the Turtles briefly, then makes its way for the portal. One of the Squirrelanoids is seen again in a background when Dr. Rockwell beats up one of the Kraangdroids. With help from Metalhead, the Turtles free the captive mutants and they escape through the portal. In "Meet Mondo Gecko", the silhouette of two Squirrelanoids are seen before Fishface's race against Michelangelo, Casey Jones, and Mondo Gecko.
- "Invasion of the Squirrelanoids"
- "Fungus Humungous" (Cameo/Illusion)
- "Metalhead Rewired"(Cameo)
- "Newtralized!" (Cameo)
- "Within the Woods" (Cameo)
- "Meet Mondo Gecko" (Cameo)
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.