Tbh let's leave the realms of science behind as science can't explain how a flying horse doesnt loom messed up due to the muscles needed to even fly. It's magic and the same could be said for the taming of a morbol, maybe a whistle calms it enough to ride, there is any number of ways it can be done.

Lore... Well twin killed that yes. If there are enough people who want it then why not? They are an iconic final fantasy mob so they have the status which is all that matters (hell in the final fantasy 3ds game i have i have a morbol mob as a party member so I'm up for having it as a mount here)