There are lots of good sites on the web showing various cow cake patterns. After looking at them, I adapted to make the cake below. I was in a hurry, as it was for a cast party (we were doing a play called "8 Cow Saint" so a cow themed cake was appropriate!).
I didn't have time to make the base cakes, so just bought 2 round madeira cakes - always good for carving and shaping.
1. Cut the dome off one round cake to level it.
2. Carve the head of the cow out of the second piece (as shown), and make ears and feet. The feet are easy - just two triangles, with the tips cut off to abut the body of the cow. Ears are roughly diamond-shaped.
3. Use a skewer to attach the ears to the head.
4. Use frosting to help secure legs, ears and head to the body. Crumb coat, then frost all over.
I used chocolate frosting for a brown cow, but reserved some white for a white muzzle and blaze on its head, as well as hooves. Eyes were Rockets (like M&Ms). For the nostrils, I just pushed the frosting into the right shape.