Slash is a mutated snapping turtle from the Teenage Mutant Ninja Turtle franchise - he appeared in the 1987 and 2012 cartoons and the Archie and IDW comics and was antagonistic towards the Turtles in all four incarnations.
Like many turtle villains he was altered from cartoon to comic, in both cartoons he is a mutated animal while in the Archie comics he is actually an alien who just happens to resemble an anthropomorphic turtle.
After Spike's accidental mutation, he and Raph eagerly upgraded their bond to a partnership, swearing to live only for each other. But Slash's new mutation was unstable, and the insane new mutant tried to kill Raph's brothers to have Raph all to himself. Raph turned on Slash for this treachery, and they once loving pair became painful new enemies. Slash was then captured by the Kraang Hive Mind, and escaped with the help of a fellow captive, the Newtralizer.