Their origin was finally explained in their villain micro. Rocksteady was a gang member. After he and his buddy Bebop were kicked out of their gang, they joined the Foot Clan and defeated a bunch of other gang members for the honor to become mutants. Rocksteady became a mutant rhino while Bebop became a mutant warthog. After screwing up a job and thinking Karai to be dead Rocksteady suggests that since Karai is dead, they return to Foot Headquarters and just say that she deserted. Karai isn't dead, however, and has overheard the entire mutinous conversation. She sends a bunch of Foot Ninja to kill them. However, Rocksteady and Bebop defeated them and ask for a second chance. Karai than decided to keep them due to their brute strength.