We're RPers, imagination is sort of part of the deal. The game is limiting on what we can use. Honestly, I don't think there's a single mount that would be "lore breaking" if you did it right. You already said you don't intend to ride Sleipnir. If you want him to be a black horse, then he's a black horse.
Obviously there are certain lines of lore that can be crossed that make you think the person can't RP in the world they're given. But at the same time, I think there is a lot more room for creative ideas that still fit with the lore.
If people are going to turn their nose up at you because you're on a mount that they make assumptions about, that's their problem isn't it? RP requires explanation. Our characters have things about them that the game can't show. Its like if a character lost their arm, but someone goes "But I see their arm right there in game."
Have fun. Be creative. That's what RP is about.
Obviously there are certain lines of lore that can be crossed that make you think the person can't RP in the world they're given. But at the same time, I think there is a lot more room for creative ideas that still fit with the lore.
If people are going to turn their nose up at you because you're on a mount that they make assumptions about, that's their problem isn't it? RP requires explanation. Our characters have things about them that the game can't show. Its like if a character lost their arm, but someone goes "But I see their arm right there in game."
Have fun. Be creative. That's what RP is about.