Mo-Lec-U-Lar

A humanoid built out of spheres with an atom on his chest, and the Mob’s answer to every problem that needs a face. What makes him work as a character is that the show gives him stated limits, and then holds to them.

The transformations run through a hand-carried Frisbee ring rather than any innate ability, so separate him from it and he is stuck. He cannot copy a victim’s mind. His shadow keeps his true silhouette. Damage taken in a disguised form still hurts him, and he has to revert to his own shape before he can change again. Most shapeshifters in cartoons get nothing like that tight a rule set, and most of the ones that do break it by the third episode.

The disguises are logged episode by episode and they are rarely people: a cigar box in Stargazer’s office, Dirty Larry, Governor Xander, Monotone, the Mirage itself, and Mon*Star. The ThunderCats wiki declines to say what he is, noting it is unclear whether he is a robot, an alien or some kind of biomechanical android, so we have left the species blank as well.

Voice. Wikipedia and the ThunderCats wiki both credit Doug Preis, which is consensus rather than a document.