Stringhalt. This horse is experiencing what's known as stringhalt. There are several reasons horses get stringhalt. One is an injury to the area where there lateral digital extensor tendon and nerve come very close to the skin on the upper part of the cannon bone. Another cause is the consumption of certain toxic weeds. Finally, we have what's called idiopathic meaning we have absolutely no idea why it happens! Treatment is aimed at calming the nerve down. This can sometimes be achieved by using

Stringhalt (Equine Reflex Hypertonia)

Stringhalt and its causes

Structural Shivers & Stringhalt — Resilient Equine