So Grimlock being a guy that turns into a Dinosaur doesn't make sense but Ratbat being a bloody big Bat that turns into... whatever he turned into on Cybertron is ok? I don't really see the difference. (edit: Griffin basically already said this)
Also, I haaaaaate the idea that Ratbat used to be a human looking guy. That's trying to make Transformers "make sense". Bugger that.
Maybe they look like Dinosaurs!We've never seen what Turbofoxes (or Petrorabbits) look like... so who knows. But just because they're called foxes doesn't necessarily mean they have to resemble foxes. After all, Firefoxes (aka Red Pandas) are neither foxes nor bears and are more closely related to raccoons, weasels and skunks (Ailuridae family).
One of them does, yeah.