The Strange Ox

Quest Start Location: Acts 1 (Emerald Grove)
To start this question, you need to get a potion, spell, or item (some can be found by looting nearby containers) that allows you to Speak To Animals, then speak to the Strange Ox at the Emerald Grove in Act 1, found along with the other livestock just north of the first vendor.

After speaking with it, you’ll next encounter the ox in the Last Light Inn in Act 2 near the blacksmith.
In Act 3 it can be found in Rivington, just west of the Requisitioned Barn. Climb the small hill and open the shabby barn door to speak with the bizarre creature, and continue to complete the final part of this missable, surprising quest.