Er, that's wrong.
Those were their Kaijin forms. Especially Kamen Riders Ichigo, Niigo, V3, and Amazon.
Why?
Every time Shocker makes a Grasshopper Kaijin, it looks...EXACTLY like Ichigo, only in different colors. Even when they're evil.
Amazon was just another beast man, only refined due to his bracelet.
V3 was made in the same method that the original two were made.
Hell, even Kamen Rider Black is technically a Kaijin Rider, as we SEE his grasshopper monster in the first episode, then see the King Stone refine that into his Rider form.
Really the heroes becoming monsters was common to all of the Showa Era, as well as Shin, ZO, and, maybe, J.
Kuuga sorta continued the tradition, though it went about it a different way, and we know where it went from there.