I know if you get black legging and black polo and black hat and sew on irregular shaped white patches, youv the cow. My little own little one had a little cow handbag that she carried (im assuming its for nativity play). Another child had a cow print dressing gown on.
Go this off a website for you:
Step 4: Make Nativity Animal Costumes
Some of the animals that were part of the nativity scene included:
Sheep - Purchase a simple sheep costume, or make one yourself using white sheets, mittens and black slippers for your hands and feet, a bell around your neck, etc.
Camel. If you cannot purchase a simple camel costume, you can make your own. Wear light brown sweat pants and a sweat shirt. Purchase a piece of light tan felt and cut out a large circle to place in the center of your sweat shirt. Paint your face light brown.
Donkeys - Dress in dark brown sweat pants and sweat shirt. Paint your face brown.
Cow - Dress in white sweat pants and white sweat shirt. Cut out a piece of pink fabric and attach to the middle of your sweat shirt. Paint your face white.