Modding Help Looking for a good tutorial(s) on how to create custom pets/monsters

Hi, I am trying to create a mod that let's a player buy a "pet"/monster which is a fish (all the same ones from the game), and place it in a body of water where it will stay unless killed. Unfortunately, I am VERY out of my depth. If you know of a good tutorial, or a couple good tutorials that could help me work through this, or if you want to help design this mod, please reply. ^_^

Any and all help is appreciated.