The story of Kensei and the dragon -- cutting out his heart and that. (It's in ep. 22 and I only generally remember what it was about.) I sort of figured that sending Ando back to Japan was symbolic of the heart-cutting-out bit, but it occurs to me that a dinosaur could be representative of a dragon, so maybe I'm off. It'd all tie together if Hiro were Kensei as ladybug218 suggests, though I'm not sure how that would work.
(Randomly, I wish the dinosaur had feathers. Not going to happen, that, but it'd be awesome and modern.)