Found confirmation of the Porg theory I mentioned a while ago.
When we first scouted Skellig Michael, the island where we shot Luke’s stuff, it was covered in puffins," says Johnson. "It’s a bird sanctuary, actually. So there were all these adorable little puffins all over the island. It was first just like, ‘OK, let’s come up with the Star Wars version of that.’
The rest of the article is good stuff too, by the way.