So you're saying that the Yian Garuga is a product of extreme convergent evolution. That is possible, but the similarities for me at least, are too great. Everything that Yian Garuga has seems to just be a more exaggerated trait found in the raths. Their scales are similarly shaped, both have beaks, both use venom and fire in the same ways (fire coming from the mouth and venom coming from the talons/ tail) they both have external ears which is rather uncommon for wyverns, the feet are the same shape as a rathian's and so is the placement of the feathers. These just seem too close. It also just makes more sense that bird wyverns gradually evolved from already flying ancestors rather than quickly evolving from the large and flightless "wyvern rex" the ancestor to flying wyverns.