Maxence (The Next Chapter)

Rocksteady is an animal-born mutant rhinoceros who is one of the "brothers" of The Foot Clan.

History
As the Turtles spy on Baxter Stockman at the zoo, a warthog, a rhinoceros, and two humans are seen trapped in metal cages. Human DNA is extracted from the humans and put into the ooze, the ooze is then spilled on the rhino and the warthog, turning the two animals into two baby mutants. The mutants are then "rescued" by Stockman and raised in the basement of Techno-Cosmic Research Institute, which is Baxter Stockman's laboratory and where Stockman created the ooze.

When Team Turtles comes to confront Shredder over his possession of the mutagen bomb, Bebop and Rocksteady are guarding the front of the Technodrome.

After the Turtles defeat Shredder and Baxter Stockman, Bebop, Screwloose, Splinter, Rocksteady, Wingnut, Leatherhead, Slash, Mondo Gecko, Genghis Frog and Man Ray go to live with April and the Turtles in the Second Time Around antique shop.