For my twins 2nd birthday I did a pig and cute 3D cow birthday cake.
I cooked the head and the body in round Pyrex bowls (smaller bowl for the head). All up it was about 3 cake mixes to fill both bowls. The legs were cut from cake cooked in a square tin. All pieces were secured by skewers and carved at an angle so they fit nicely together. This was a bit of trial and error.
I did a crumb coating (thin layer of icing) over the whole cake and placed fondant over the top. Each part was done separately but it looked great in the end. The ears and horns were made out of fondant which I left overnight to dry into the correct shape. The hair was cut from rolled fondant. The eyes were made out of fondant too and stuck on with a little bit of icing used like glue. The pink nose was stuck on and the nostrils and mouth were created by pushing the back of a knife into the fondant.
This was the same method for the feet of the cow. The spots on the cow were cut out of rolled fondant and stuck on using a little bit of icing like glue.
It was an absolute hit and definitely worth the effort.