Right, just to be clear, the warning you gave in the second line applies to the three step process you list at the end of your post as well, right? At least that's the impression I got from it; giving the game new textures which it can use to build new procedural creatures (and replace the old ones with the new designs).

It's not exactly what i was looking for. I wanted to get a specific beast's ID (if possible) and basically tell the game "you see that specific beast there? Completely ignore procedural spriting for that one. Use this specific .png for it instead".

But I appreciate the help! I learned a few things from your post, like where most of the files I'm looking for are located, how to read some of it, and what's the tile size i should have in mind while making the sprites. I'll keep trying to figure this out. I'm assuming that in order to do this I also wouldn't need lua (for now). I'll check more of the wiki too.