This costume starts with a sweatshirt two sizes bigger than your child's size -- found at a discount store for about $5 apiece. You'll also need a hot-glue gun and scissors.
1. Trim the sleeves of a large tan or brown sweatshirt to your child's wrist length; fold in and glue raw edges. Cut and glue brown felt spots to back of sweatshirt.
2. Tail: Cut one end of a skein of dark-brown yarn. Lay yarn out in a straight line. Loop brown pipe cleaner around middle of bundle of yarn and twist ends together. Hold skein by twisted pipe-cleaner ends (it should dangle like a ponytail). Insert pipe-cleaner ends into back of sweatshirt. Bend them flush with shirt and cover with a small, glued-on piece of felt or cardboard (this reinforces the tail). If necessary, trim tail so it doesn't drag.
3. Mane: Cut another skein of dark-brown yarn into four sections. Tie a piece of yarn around center of each section. Glue three of these bundles to the center of sweatshirt hood, front to back.
4. Glue fourth bundle to brown cap. Cut and glue large brown felt teardrops to cap for ears.