Definitely possible!
I think it would be quite restricted when roleplaying with others, though? Firstly, very few people have the PvP horse mount, so if you planned on teaching them to ride a horse or something it would be quite difficult to find people who could particpate. I also don't know anyone who RPs PvP directly. So while your character could be a Grand Company horse trainer, it would be a little limited when interacting with others aside from Serena talking about it, if that makes sense
Or at least - I can't think of anything, but I imagine you have many more ideas than I! BAn interesting idea and I'd love to see where it could go 
As far as the elemental horses go, that would be a much more difficult route to take. From my understanding they definitely aren't an egi, they are Nightmares, sacrificed from the beast tribes to their respective Primal, except Shiva's mount, which was a sacrificed Unicorn from her followers. If you decided to portray them as egis, it would require your character to be a Summoner. Hopefully someone could come and shed some light on Summoner lore, as I've read a few different interpretations and don't know right from wrong.
It would be received quite subjectively: some would roll with it, others who are more strict when it comes to lore may not be so fond of the idea.
Personally, I do second the idea that you go in the Chocobo Trainer direction. Your horse training/racing knowledge could easily be applied to chocobos, with a few tweaks. Chocobos are the Eorzean version of horses, and everyone could join in with it as OOCly we all have a Chocobo mount.
I think it would be quite restricted when roleplaying with others, though? Firstly, very few people have the PvP horse mount, so if you planned on teaching them to ride a horse or something it would be quite difficult to find people who could particpate. I also don't know anyone who RPs PvP directly. So while your character could be a Grand Company horse trainer, it would be a little limited when interacting with others aside from Serena talking about it, if that makes sense
Or at least - I can't think of anything, but I imagine you have many more ideas than I! BAn interesting idea and I'd love to see where it could go As far as the elemental horses go, that would be a much more difficult route to take. From my understanding they definitely aren't an egi, they are Nightmares, sacrificed from the beast tribes to their respective Primal, except Shiva's mount, which was a sacrificed Unicorn from her followers. If you decided to portray them as egis, it would require your character to be a Summoner. Hopefully someone could come and shed some light on Summoner lore, as I've read a few different interpretations and don't know right from wrong.
It would be received quite subjectively: some would roll with it, others who are more strict when it comes to lore may not be so fond of the idea.
Personally, I do second the idea that you go in the Chocobo Trainer direction. Your horse training/racing knowledge could easily be applied to chocobos, with a few tweaks. Chocobos are the Eorzean version of horses, and everyone could join in with it as OOCly we all have a Chocobo mount.
