There has only been one horse that I didn't get along with. I tried and tried...but that 'connection' was never there.
She's a 13yr SSH mare we had a few years back. We had her for about 4 years, but she was just soooo nasty I couldn't stand to be around her. Once you had her haltered she was fine, but in the pasture or stall...ugh.
She came from a home of inexperienced horse owners. They thought it was cute when she would nip, or funny when she would ear back and slam the bucket when you were pouring her feed. I think the real issue with her came when she foaled and was VERY protective of her colt. She'd kick and charge at you...so they didn't mess with her. Once the colt had been weaned, the damage had been done.
We corrected a lot of her people problems, but she still had that nasty mare syndrome lol. She wasn't that bad, but she was just so testy I couldn't take it.