1) Armor for the rump of a horse 2) Strap 
A crupper (ər; occ. spelled crouper) is a piece of tack used on horses and other equids to keep a saddle, harness or other equipment from sliding forward. ==Construction== The crupper consists of a loop (the crupper itself) and an adjustable strap (crupper strap or back strap) that connects the crupper to back of a riding saddle or the other part...
strap of saddle that passes beneath horse's tail; armour for horse's flanks
• (n.) A leather loop, passing under a horse`s tail, and buckled to the saddle to keep it from slipping forwards. • (n.) The buttocks or rump of a horse. • (v. t.) To fit with a crupper; to place a crupper upon; as, to crupper a horse.
Crup'per noun [ French croupi...re , from croupe . See Croup the rump of a horse.] [ Written also crouper .] 1. The buttocks or rump of a horse. 2. A leather loop, passing under a horse's tail, and buckled to the saddle to keep it from slipping forwards. Crup'per transitive verb To fit with a crupper; to place a crupper upon; as, to crupper a horse. 
A leather loop which passes under the horse's tail and is attached to the
A crupper was a piece of armour worn by a war-horse to protect the hindquarters.
[n] - a strap from the back of a saddle passing under the horse`s tail
noun a strap from the back of a saddle passing under the horse`s tail; prevents saddle from slipping forward
Plate defence for a horse
a leather strap fastened to the saddle of a harness and looping under the tail of a horse to prevent the harness from slipping forward. See illus. under · the rump or buttocks of a horse. · armor for the rump of a horse.