I was thinking that, when critters die, they drop a certain item. So if you were to kill a squirrel, for example, there would be a 50/50 chance of getting the essence of it. In this case, the squirrel essence would let you make a potion or equipable thing that lets you climb up walls for a short period of time. The critters would drop them in pre-hardmode, so the squirrel potion/equipable thing would only be useful early in the game.