Acepromazine is one of the members of the phenothiazine class of drugs. Back in my young days as a hospital resident, we routinely ordered another phenothiazine drug almost identical to acepromazine, chlorpromazine (Thorazine) to treat postsurgical nausea in patients who were a high risk for aspiration, such as facial trauma patients who had their jaws wired shut. I recall extrapyramidal side-effects being not uncommon. Manifestation was uncontrolled jerky muscle contractions of the limbs which were quite distressing for the patients, and even more so for the relatives who were present.
I can imagine a drug seeker working around a horse barn rummaging through the horse meds in search of something to take for themselves, and deciding that since Ace is used as a tranquilizer it might be worth a shot for themselves. Anyone with some pharmacology training though, would know that there are different classes of sedative/tranquilizers. For example Ace and Thorazine are pharmacologically very different from the benzodiazepine class of sedatives, like Valium, Xanax, and Ativan.