My family had a well trained, well traveled and shown Arabian stallion.
I absolutely believed he has PTSD.
One time, he was involved in a bad trailer wreck where he was on the left side of the trailer, where the trailer flipped and went off road and the other horse was dead and laying on him for several hours before they could cut open/remove the deceased horse. Now the stallion would load just fine in a trailer, but put him on the left side and a few minutes down the road, he would begin being extremely agitated, sweating, and basically panicking. Right side, loose, slant, whatever, and he was fine.
This same stallion had also been handled by a not-so-nice woman. She drove a particular vehicle and whenever Gay would see the same VW Baja, he would pin his ears, chase after it, and just generally be agitated. Same if he actually saw this women...
Otherwise, this was an absolutely trustworthy horse and probably the easiest one to ride. Ride him in the city, move cattle, whatever. Gentle with mares and foals. But just put him in a couple of circumstances and he absolutely couldn't take it.
It doesn't take much.
I don't know who haltered my horse as a yearling, but if I put her in a training halter and stand her up with a whip and make an aggressive posture towards her, she flies backwards. I had no idea she had been schooled in halter and a person that knew her, told me she had. Thought it would be fun to bring her to an open show. Not worth it to cause the horse stress. I just showed her gently with treats instead. Not going to be a great look in an open show, but does it matter? I'd rather her not be sweaty and agitated by asking her to do something that she had been last schooled in about 17 years prior....17 years folks...