Undershot: A deformity in which the lower jaw projects beyond the upper.
Unsoundness: Term used to describe any condition, or conformation fault that limit the horse's ability to perform his job. Including sidebone, ringbone, roaring and others.
Up to Weight: Term used to describe a horse that, by virtue of its size, substance and conformation, is capable of carrying substantial weight.
Urticaria: Allergic reaction characterized by bumps or weals on the skin. More commonly called hives.