I noticed that the dog model I am remixing had messed up legs. The front legs have large concave indents and they lack holes to attach them to the body. However, the cat model (by Lohman) the dog model is remixed from has no such issue, and the legs are otherwise identical.

No hate to you, Balik0; you changed a lot of cat stuff to plausibly look like a dog, which is a lot more than I know how to do. I haven't modeled or programmed in several years, but I have more experience programming and I imagine the issue with the dog's legs is just as easy a mistake to make as adding an extra semicolon that causes the whole program to crash.

Anyway, I used my VERY limited knowledge of Prusa3D Slicer to cut the dog model and the cat model in two pieces, then add the cat legs to the dog body, so now the dog has 4 working legs and all is good.

Be careful when moving around the model: I think I properly connected the two parts, but they still can be highlighted separately, so something I haven't predicted could probably happen! Just be sure both pieces have the same width (height?) and are touching, and you should be good.

My next objective is to figure out how to make the dog's and cat's necks smaller, so they don't poke out of the shoulder so weirdly (again, no hate, this absolutely isn't easy).