In fact, the mere existence of the Mutant Turtles was proof that other ‘mutanimals’ could be created as well–resulting in the villainous Bebop and Rocksteady along with a number of other mutants created in the wake of the Turtles’ exposed existence.

Source: https://screenrant.com/tmnt-first-mutants-explained-triceratons-dinosaurs-idw-publishing/

In the latter, Bebop and Rocksteady are part of Superfly's mutant family.

Source: https://screenrant.com/tmnt-mutant-mayhem-bebop-rocksteady-differences-michael-bay/

It may sound weird to consider a group of three old grumpy men as interesting, but they're arguably the most recurring characters in all of the anime outside of the Bebop crew themselves.

Source: https://gamerant.com/cowboy-bebop-most-interesting-side-characters/