A horse's hind legs can become swollen for a number of reasons. Symptoms range from mild to severe; it is imperative to contact a veterinarian promptly to determine an effective solution for bringing down the swelling.
The stifle on a horse is a joint that controls mobility in the legs. A fractured stifle is one cause of swelling in a horse's upper thigh, according to The Merck Veterinary Manual. To determine if this is a cause of swelling, a veterinarian will need to take an X-ray of the horse's limb.
Bog Spavin is an equine condition in which the hock joint becomes inflamed and filled with fluid due to stress, tear or other injury. The overproduction of fluid to the joint creates a swelling affect in the upper thigh of a horse. With Bog Spavin, the fluid must be drained to reduce swelling and discomfort.
According to the Paragon Veterinary Group, curb is a condition that causes horses' hind legs to swell due to a damaged ligament. Inflammation is apparent as new bone growth begins around the weakened area. Swelling due to curb can settle on its own.