Maybe they picked up Earth television broadcasts of old sword and sandal movies and though "Hey! Cool!" at the creatures, so they reformatted their alternate modes to appear like the monsters.
I've been wondering why such monsters were branded Autobots in the first place. Did Hasbro see all the monster Pretenders and Headmasters (who were nearly all Decepticons) shelfwarming and wonder how well monster Transformers would sell if they were Autobots?