VGResource Pokédex Project: IDEA PITCH

[Sketchasaurus]

It could instead of becoming an Iguanodon, evolve into something more aquatic in appearance/less plain (Iguanodon is not as visually interesting as other dinosaurs in that group)
If the Iguana goes a dinosaur/prehistoric reptile route it could potentially become something like the following:
Ouranosaurus, Parasaurolophus, Corythosaurus, Lambeosaurus
Spinosaurus, Baryonix, Suchomimus (though I'd avoid these because a: Spinosaurus is somewhat cliché now, and they all look like crocodiles)
Dimetrodon
Plesiosaurus
Mosasaurus
Liopleurodon
Or, if all else fails, you could base it off of the american Godzilla which, in the movie, is heavily implied to be a super-mutated marine iguana.