Correct, the difference is Big Belly enhances the vanilla art whereas Big Booba only has large belly for big breasted Lona.
You can also edit them directly in
Cheat Framework's Status menu:
View attachment 5600797
Just note that what you add will probably block your original path that you were working towards. If you mainly want to access locked equipment, another option is the "Equip Anything" cheat in Game Tweaks.
Just tweak the JSON for that animal to meet your needs. Could set killer to Sex = 999 and Fucker to Sta > 0 (that way they - in theory - stop when you're exhausted)... or just HP > 0 like the Wolf's default for killer.
The main post is misleading - there really isn't any need for MTL unless you are trying to access the incomplete sections of the game (one map, last I checked).
It might be your folder name. Ruby, the language RPGMaker is based on, doesn't play well with some special characters.