I dont think bokoblins will do much damage to your horse, but I'd stay away from Lynels and Guardians unless you're very confident in your horseriding.
I've tried my hand at battling a Guardian on horseback once. Got hit with a laser but my fucking horse took it look a champ. Just stood there, while on fire. Meanwhile Link was writhing in his death throes.
You really need to give more context before spoiling end-game content.
More context on that spoiler dude :/
I thought me bolding the person's response would be enough context. I'll change it.