“From what I gathered, Rian [Johnson] had gone to shoot this sequence on Skellig Michael, which is a real island location that stands in for Ahch-To, and that island is covered in puffins,†said creature concept designer Jake Lunt Davies speaking to StarWars.com. “It’s a wildlife preserve and everywhere you look, there are hundreds of birds dotted around the landscape. From what I gathered, Rian, in a positive spin on this, was looking at how you can just roll with it, play with it.â€

You can’t really have earth creatures popping up in a Star Wars movie, aside from, you know, humans, so Johnson wanted his own indigenous creature. So Lunt Davies and co. made the Porg. It seems it’s easier to get the little CG bird-hog things into the picture than puffins out.
Now, look back at Return of the Jedi and wonder what it would be like if there were little bipedal bears running around the Redwood Forest that George Lucas had to account for.
Article source: https://www.technobuffalo.com/2017/12/22/star-wars-last-jedi-porgs/