A Splint

1. A narrow strip of wood split or peeled from a larger piece.
2. (dentistry) A dental device applied consequent to undergoing orthodontia.
3. (medicine) A device to immobilize a body part.
4. (military, historical) A segment of armour consisting of a narrow overlapping plate.
5. (mining) Synonym of splent coal
6. (zootomy) A bone found on either side of a horse's cannon bone; the second or fourth metacarpal (forelimb) or metatarsal (hindlimb) bone.
7. (zootomy, veterinary medicine) A disease affecting the splint bones, as a callosity or hard excrescence.

For a horse to pop a splint is for it to receive an injury to the splint bone or surrounding area.