Anything that won't look after itself (forget the rider) if it looks after itself regardeless of what it is doing, then you have half a chance of being OK.
It is when they through themselves down that I will quite happily give up with them!
Don't care if they don't look out for me but they have to look out for thmselves.
Had one that hafter double barreling me in the stable for putting hoof oil on. When I returned to ride him, he started napping. Wouldn't move in or out of yard. Then he started to go but then would through himself upside down in a ditch with you under him. Not fun. I never sold him as I couldn't have him on my concious damaging someone else.
A horse that bucked big and maliciously - trying to get you off. And something that reared regularly high enough to go over also.
To be honest I think I would not want to own any horse who would behave in a malicious fashion towards the rider on a regular basis - to be a decent partnership you have to work together and I would just not want to own or ride any horse who had a nasty spirit in that way.