Stocking up, also known as Filling up or Filled legs refers to a condition in horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s caused when fluid builds up and swells the leg. Sometimes it is linked to lameness

Lameness (equine)

Lameness in horses and other equidae is a term used to refer to any number of conditions where the animal fails to travel in a regular and sound manner on all four feet...

 and other times it is purely an aesthetic problem. Fluid buildup can be caused by dozens of different things, though often is seen in horse standing in a stable or horse trailer for long periods of time. Standing after inadequate cool down following hard exercise is a common source of filling up

Hind legs are more often affected than front legs, usually on both sides. If there is swelling in only one leg, there may be another problem such as an injury to the limb or an abscess

Abscess

An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...

 in the hoof

Horse hoof

A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

.

Filled legs with no injury or lameness may go down with light exercise. Bandaging with a stable bandage

Stable bandage

A stable bandage, or standing bandage/wrap, is a type of wrap used on the lower legs of a horse. A stable bandage runs from just below the knee or hock, to the bottom of the fetlock joint, and protects the cannon bone, tendons of the lower leg, and fetlock joint.-Uses of the stable...

or "standing wrap" can help prevent and reduce the swelling.