Why did the Bearger show up in base game?

I have a long running save file that in the save menu shows me it is DS. It is not RoG, SW, or H.

I am 99% sure I made this file base game with SW compatibility and nothing else..so why did the RoG exclusive boss show up to my base?

For extra info - there are no birchnut trees, catcoons, nor tumbleweeds but there are mini glaciers and moleworms.

Is my save just horribly bugged?