Frosty is a giant mutated ice monster. He appears in the episode The Ice Creature Cometh of the 1987 TV series.
Frosty was first made when Bebop and Rocksteady dumped a vat of mutagen onto the ice outside the Technodrome. Because of his unusual origins, Frosty was the size of a building and not particularly intelligent. Initially hostile to all inside, he became attached to Bebop after the mutant removed a missile embedded in his hand.
He was sent along with the other two mutants to retrieve thermal explosives from New York, during which he froze all the buildings he came across. When Bebop departed without him, Frosty was grief-stricken and began trashing an army base. The Turtles managed to trick him onto a rocket and send him back to the Arctic, where he melted completely.