I'm not to keen on behemoth (hybrid or not) being some kind of playable race. Behemoths have always been an enemy of sorts, usually an extremely strong enemy through out FF history. I just don't see any lore supporting this specific type of hybrid in the FFXIV:ARR or any FF lore. Hybrids exist through out FF the series itself, but a behemoth? I can see a dragon type race being included at some point, I'm pretty sure Yoshi said he wanted to eventually go in that direction.

Just my personal opinion, I'm not sure it would effectively work well in Eorzea, plus it's just not that appealing.