I think it is probable that the manga has been categorized as shounen-ai by error, because the mangaka mainly writes BL. Plus, sometimes the categories they put here are wrong, so it may not be shounen-ai but just seinen, or it may even be shoujo (with that girl Touma-chan)
If the horse really turns into a person... if Akira falls in love with the horse in his human form, not as an animal, and kisses the human Romeo, it wouldn't be too hinky. I mean, it could be a human who was cursed, like the tale of the princess and the frog.
Plus, Hideyoshico has done a lot of stories with "weird" things, like a guy falling in love with his crazy stalker, a bishounen who turns to be a sadist, a guy being possessed by his dead girlfriend, and some hinky things too like travestis, a nephew lovesick with his uncle who raised him (but i don't know if this last one ends as incest or not)...
The magazine that it's released in is all seinen. I wonder if it's been categorised as shounen-ai just because Hideyoshico is primarily a BL mangaka?
If you've read any other manga by Hideyoshico, I don't think you guys would think she was going to have the horse turn into a person... That's pretty weird, and kind of a strange kink for her. I'd be super surprised if that's what happens.