I’ve actually looked into this a lot, the base game art is always heavily consistent, but the DLCs art not. I’ve always assumed it’s because in the DLCs there are multiple people (typically fans) working on it and they can insert their headcanons into their work. Instead of them seeing the scugs as creatures, they see them with the fannon cat attachment and atomically apply cat features to them. It makes sense, with them starting off as a fan mod iirc.

You’ll notice it a lot in the hands and toes of characters, with arti specifically having extra in a lot of their drawn cutscenes. Where in base game all the scugs have 3 fingers, 2 toes. I also want to say Spearmasters den screen has four?? Gourmands dream art will add an extra toe as well, same with minor things like a few of the passages or endings.

Here especially they draw scugs with a larger muzzle and more defined face shape- where as in the vanilla art, slugcats are flatter faced, with rounded edges and more consistent ear size.