Behemoth mount is canon, read the flavor text on it. My character uses a coeurl, which is also canon. There are plenty of black mage type/void callers in RP who use the ahriman mount, so I don't see this as much of a concern either, just so long as the players using them understand that there's a stigma attached to summoning things from the void.
As far as things like Sleipnir, don't worry about what other people think. Just say it's an impressive-looking horse and call it by a different name. I'd honestly be more concerned with the horse mounts which drop from Ex Primals. Sure, there's flavor text to explain those as well, but not everyone can be a Primal slayer.
As far as things like Sleipnir, don't worry about what other people think. Just say it's an impressive-looking horse and call it by a different name. I'd honestly be more concerned with the horse mounts which drop from Ex Primals. Sure, there's flavor text to explain those as well, but not everyone can be a Primal slayer.