The baby sign for horse is like a horse twitching an ear to brush away a pesky fly. You start by taking your dominant hand with thumb, middle finger, and index finger extended and making an L-shape, with your other two fingers tucked in and palm facing forward. Put the thumb on your temple, and move the index finger and middle finger down and up together.
Most babies love animal games. So make a game out of identifying pictures of different animals by making the appropriate sign.
Make a horse's noise as you teach your baby the horse sign. It will make teaching the sign fun and much more memorable.