Take a toy rubber or plastic mouse/rat and mold pieces of aluminum foil by pressing the foil over the top of the rat, forming a foil cup that has the shape of the rat on the inside, leave the bottom open and pull the toy rat out. Make a bunch of these aluminum rat cups. Use some additional foil to form legs so the molds sit open side up on a cookie sheet.
Spray the inside of the foil rat molds with non-stick cooking spray and fill with chocolate cake batter following the instructions on the box. Leaving the molds on the cookie sheet for support, bake the rat cakes in the oven, again follow box instructions, you may find it helps to over bake them a bit.
When finished baking, let cool and pull off the foil mold and trim the bottom of the cupcake rat with a knife so it sits flat.
Use red cake icing in the little squeeze tubes to inject (from the bottom) the insides with a little gooey 'blood' use licorice whips for a tail and red cake decorating beads for eyes.
You usually will only get one rat per mold so make plenty of the foil molds, they're easy to do. You can make these into bats by adding wings made from Fruit Roll-Ups and leaving off the tail.