It’s an alpha 2 agonist under the class of clonidine derivatives, not a phenothiazine. You’re probably thinking of acepromazine.

Xylazine’s a sedative. We use for general sedation on its own (with or without butorphanol), and as a premedication prior to induction of general anesthesia with ketamine to bypass the excitability phase of ketamine in large animals. I usually add on midazolam or diazepam in adult horses with ketamine as well (it also can be excitatory in awake adult horses without sedation).