Robonoids were once Tyhrranoids before their transformation into machines by Nefarious via his Biobliterator.

Robonoids were much smarter, and caused much more damage to Ratchet. They were also more difficult to fool with the Tyhrraguise. They could take more damage than the normal Tyhrranoids.

When Big Al discovered the means to transform victims of the Biobliterator back to their former organic forms, it appears most of the Robonoids chose to return to their former Tyhrranoid forms. A few apparently chose to stay Robonoids, but its likely their numbers are quite small.