For my part I see the Dinobots and Predacons (and other similarly not-'in-disguise' groups/TFs, like the Terrorcons) as more 'assault weapon/mode' forms drawing/modeled on local life forms, rather than 'disguises' per se. A giant, nigh-invincible, fire-breathing, robotic T-Rex isn't exactly going to fool anyone disguise-wise, but it sure as heck will scare the crap out of anyone in it's way.
Same thing for a huge mechanical eagle that suddenly swoops out of the sky, picks up a freaking tank in its talons, and then casually tears it in half with its beak.

Lions and tigers and bears, oh my buffalo are pretty deadly creatures after all...it makes sense to me for a form-adaptive species of robots to adopt-and-adapt even just the basic shape/design of the more deadly/powerful local life forms for their war machines, even without the disguise aspect.

It's a lame excuse/reasoning I know, but works well enough for me.