Apparently if there is a Miqo'te in your party in Skalla, and they are under the Well-Fed buff (ie having eaten something), the second boss will have a Fat Cat inside.

Like, this is an actual thing reported and tested by Reddit. I've never been able to check this myself, because I'm usually too busy healing (that boss doesn't even have unavoidable AoEs, why are you still taking so much damage), but I think there are screenshots floating around.