you can use any cake mix to bake the 2 piece lamb or bunny pan set. I've found yellow or pound cake works the best.
Tip # 1:
cut a chopstick or dowel rod that is the height as the pan at the area from the top of the lambs head to the base lay this on top of the cake batter before you bake, close pan and bake. (the cake batter bakes around the dowel rod and then when you stand the cake upright to ice you will have a little "backbone in your cake" and will help after you ice the cake so that you can move the cake (even in a car).

Tip # 2:
fill the half of the lamb pan without the hole as full as you can with cake batter, then put the other half on top THEN: take a thin piece of wire or even a piece of string and tie the two halves together. You can even put several garbage bag twist ties togehter to make a "string to twist around the pans"

Tip# 3:
During the last 15 minutes of baking turn the lamb pan over so the side with the hole is on the bottom. The battter will not leak out, and this will brown the front of the lamb, give definition to the marks on the front of your lamb cake and make the crust a little sturdier.