HORSE: The American Sign Language (ASL) sign for "horse"
 

The sign for "horse" is made by forming the letter "u"  (or "h") with your dominant hand. Place your thumb on your right temple, or a little higher. Bend and unbend your first two fingers a couple times.

HORSE:
 

Sample sentence: HORSE, YOU RIDE-ON BEFORE?

If you are discussing a PONY one way to convey that concept is to sign SMALL + HORSE:

If you are discussing a HORSE FLY one way to show that concept is to sign HORSE and fingerspell F-L-Y:

If you plan on going horseback riding and need to discuss HORSE TRAILS here is an option:

If you want to learn more animal signs, check out:

Notes: 

If you use all the fingers instead of just the index and middle finger this sign means "donkey."  If you sign "donkey" but only move the fingers from the upright position to the bent position (one single, quick motion) and don't move them back--then you have signed "stubborn." -- Think of how some animals bend their ears down when they are behaving stubbornly. 
See: DONKEY

Also see: ANIMALS