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Ar, but I don't think everything should be done in a template-like manner. Perhaps, yes, a Terror Knight on a dinosaur is an exceptional phenomenon in Heroes. But we need to introduce some originality so that the heroes stand out from other fantasy games. Although dinosaurs underground are an unseen wonder, they would be more suited to tropical jungles. Hydras, according to mythology, should also inhabit swamps, not dungeons. But is it worth stubbornly adhering to mythology? Of course, this is a violation of the copyright of the ancient Greek people :), but in that case, the hydra should be an uber-creation, which can only be killed with cunning (remember the 12 labors of Hercules ;) )
P.S. I agree that reptiles don't have the best place underground, but it's still not nonsense, IMHO.
P.P.S. On the verge of nonsense)
P.P.P.S. And the idea of a rider on a dinosaur is, IMHO, very original. I like it. Especially, I like how viciously the dinosaur roars in campaigns and scenarios :)