Still in Act 1, so no spoilers please.
I found out that if you have the Idol of Silvanus in your inventory and transfer it between characters while in sight of the Strange Ox, the entire herd aggros and attacks you. Cue the speculation!
Edit: Did some testing, apparently all the tieflings react the same way, so it’s not some scripted event for the ox :(
It’s also in act 2.
Details (mild spoilers)
It’s in the stables of the Last Light Inn. The loot if you kill it is the same as if you kill it in act 3 (better loot than if you killed it in act 1). Though if you kill it you miss out on an act 3 quest.
I recommend not killing it, and instead helping it when asked. I don’t know how to do spoiler text but there’s a good reason to help it in act 3 come final fight time