Here is an utterly easy idea.
Here is my little cow. This costume doesn't require much. Long underwear that you will draw cow spots on with black permanent pen. A bell on a ribbon around the neck, and a ribbon for the tail pinned on to the bum. The hardest part is the utters. Use baby bottle nipples and either hot glue them to the front of the pants or you can also sew them on. Use clear thread and sew right through the rubber.
Here is another use for those baby nipples. How about a pig. Use a pink one piece sleeper. Sew or glue the nipples on and don't forget to glue a curly pink pipe cleaner on for the tail. Use a piece of elastic and pink felt to make some ears.
It's as easy as one, two, teet.