I don't know what it is but when I look at Xerneas, Virizion pops into my head. I think it's because they have similar body structures but I know they are completely different to each other. With Yveltal it's the same but with Terrakion, it's because of those horns on Yveltal head. I'm not saying their uncreative, I do think the designs are awesome, but when I look at them those two Pokémon come to mind.
I'm quite curious as to what their Types would be. Yveltal definitely screams Dragon, but what it's other type could be I'm not sure, I guess it could be just Dragon, but I doubt that. As for Xerneas I think that's definitely a Grass type paired with something else, I don't see this one being part Dragon type, so I have no clue.