I’ve heard of horses that had a severe blow to the poll becoming unmanageable and having to be put down.
I would imagine anything that can cause brain damage in a human can do it to a horse.
We had a dog with amnesia- she had no short term memory whatsoever, if she didn’t see you for 48 hours (her cut off) she didn’t know who you were and had to be reintroduced. She was perfectly trainable but she just couldn’t remember people or situations. I don’t really remember the details but the girl who got her (from the pound) was a vet student and researched it a bit. Apparently if she had been a human it would have been a classic case. the funny thing was that our dogs who started out very impatient with her started taking care of her and “reminding” her of stuff she forgot -like who to bark at and who lived in the house. It was weird. I don’t know if she ever got better…
Another roomate had a brain damaged Border Collie who also had OCD but that is a whole 'nother story…