I've had one vanish recently.
Had a two star boar, caught and tamed.

The pen was regular walls on the inner pen it was placed in and another round pole fence <points on top> around that with a moat outside of the wall.

Named the boar "Original" so I would know it was the first one.

Brought in another boar tamed and bred.. have a good group of two star only.

Most were roaming around inside the round pole fence but Original was the only one left in the center pen with regular walls.

Came back one day and he was gone. No remains. Not in round pole fence area or space outside of that or in the moat outside of that.

I did just put a roof up and wondered if it had teleported to the roof or something but didn't see it.

Still a mystery where it went, like it just glitched and fell out of the map or something.