How would you handle the taming of an elite owlbear?

Running a very heavily modified Kingmaker game and the PC's just put down the Stag Lord. Well, they arranged for Beaky to put him down actually. I made Beaky CR5 and laid a Fey template of my own design on him. What happened is of little importance, what is relevant is that the cage he is in has a cold iron wrought door and after devouring the Stag Lord (and the PC's escaping to the cellar) Beaky went into his den to sleep, and the PC's snuck up and arranged the cage door to be closed again. Now they have a captive owlbear and the party's Kobold priest of Sarenrae with a high nature skill wishes to tame him.

I don't mind the idea, but am at a bit of a loss on how to accomplish it, and what to make it look like in game. Beaky will never be a pet, it's still too much of a monster, but what role could a tame owlbear play in the game? Adventuring companion is absolutely ruled out, I have 6 players, they do not need an oversized flamboyantly colored owlbear to further complicate combats and social encounters. Also, even tame, the disposition of an owlbear to my mind is like that of a hungry bear that thinks you smell like a steak dinner.

Ideas? Suggestions?