jillybean: that's very true that natural selection would most likely eliminate the horses that are born mentally "disabled", and that nowadays careful breeding makes genetic disorders less likely. However, in humans, is it not possible for two very average people to have a child which is gifted, or who has a mental disorder, however severe it may be?
Despite breeding selection, every now and then there must be an occurance like this in the horse world?
Or, as JustDressageIt said, due to horses not being self-aware beings, is the likelihood of this reduced to the point of it being unheard of?
So many questions!! lol my brain is working overdrive right now
I think the lack of self-awareness prevents horses from having certain emotional disabilities, like thinking something is wrong with them or the world around them (i.e. bipolar disorder, OCD, etc). However, I wouldn't put it past them to develop depression, as its seeming equivalent has been observed in animals. It's something called "learned helplessness" - basically, after trying over and over to escape an unpleasant situation, they simply give up and stop trying, even when the possibility of escape becomes available. The behavior of animals who have learned helplessness is basically identical to the behavior observed in people with depression. I'd bet horses could develop this type of depression.
It would be extremely rare for any genetic disorders in humans (which there is a genetic component, however small, in almost every disorder) to appear in horses spontaneously. It would have to be a very recessive (less likely to be expressed/affect the animal) gene and stay well enough hidden not to be bred out, and even then it would take both parents having the recessive gene for the offspring to express/affect the animal. Sorry if this gets into psychology jargon - I'm trying to put it in reader-friendly terms for everyone lol.
As for two average parents to have a child with a disorder - It's not always obvious that the disorder has a genetic component in that family. However, if you take statistics from every child with that disorder, then look at their families and especially their siblings, a genetic component seems to appear even though it's hard to explain. For example, with autism, we know that it's not entirely genetics and we can't seem to find a clear pattern of how the genetics influence the disorder. However, statistics tell us that if a family has one autistic child, the likelihood of having a second one greatly increases, signifying a clear genetic component.
A limited gene pool, which is both an issue in humans and domestically bred animals. In human history, there are many instances where royal families or family clans only married and had children from their same social group, leading to a very limited gene pool and making it much more likely for disabilities of many kinds to appear due to everyone carrying the same "bad" genes. In the wild, animals generally do a good job avoiding inbreeding and only the best of the best are able to mate, whereas domestic breeders of many animals have unfortunately picked certain traits, generally appearing in only one bloodline, and bred relatives together creating a very limited gene pool and allowing for more defects to appear. This is why purebred dogs are prone to various disabilities depending on the breed. In horses, I know Friesians have been inbred as well, but most other breeds have come from a variety of bloodlines and therefore eliminate the chances of disabilities occurring.