Okay, so I recently -- and randomly -- remembered a scene from an animated movie I watched when I was a kid. So I'm guessing it's from the '80s. I can't remember what it was about (at all ...) I just remember this one scene where a guy in a castle painted an ordinary horse to look like some other horse (the horse was special in some way) and then he put either a wasp/bee or a fly into it's ear so that it would be driven insane and it'd attack some guy. I remember the horse chasing someone up some spiral stairs.

Please help me remember it. It's been driving me mad. Also, my sister seems to think I've dreamt up the whole thing, but I'm sure I watched it with her, hehe.