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 :(
You’ll be able to meet him again later in the game and might learn something more about what’s going on with that ox.