^ Hells yeah!
Hmm, well we know that Amaimon was warned by Mephisto not to expose too much of his power when he went all lizardy to beat up Rin, which suggests that he has a "true form" that he can only use in Gehena. Likewise one of the Question Corners in the manga revealed that Mephisto can turn into something other than a dog, implying that he also has a "true form".
It makes sense that the God of Earth would have a lizardy body, but I wonder what the other demon kings would look like. Maybe that's where Rin's ears and tail come from.