
They both were supposed to be of the same class. Lions are kings and horses are considered majestic. That's how they met all those years ago because of a high class party. A donkey probably wouldn't have had the same status. Yes, he was also considered high class because he was thoroughbred. If that was simply the case, thoroughbred donkey doesn't sound very classy either.
Shishidou wasn't supposed to be very prey-like too. They did state that he could defend himself better than other herbivores.

I made that reference to rabbit x wolf because that's the pairing from the original manga this one came from (this is a spin-off), which is this one: http://www.mangago.zone/read-manga/ookami_kun_wa_kowakunai/
Pretty cute, I liked it, it has like a sequel or something.
Everyone here talking about the 4 ears and I've actually been wondering why the author chose to make Toshimasa a horse. I mean, rabbit x wolf makes sense, the whole odd couple of opposites dynamic and they look cute in their animal forms, etc. but lion x horse? First, as a couple, they'd look very awkward together in their animal forms as horse and lion, they aren't even from the same eco-systems, I think, not that it matters, I guess, but still. Then, the lion thing, predators, all they do is have sex and lay around, ok, I get it, good choice for Ouga's character, but why a horse?? Isn't horses' whole thing... well, their "thing"? He doesn't even use it, so what's the point? They could also milk the "he's gonna ride him like a horse" line, but no, it's the opposite, this is a horse doing the riding. What attribute did the author actually took advandtage of from horses? Could have been any other. A fucking zebra, for example, at least that's one thing lions are known for eating. Maybe author didn't feel like drawing the stripes, who knows.