Noun
The cushionlike projection, bearing a tuft of long hair, on the back side of the leg above the hoof of the horse and similar animals. Also, the joint of the limb at this point (between the great pastern bone and the metacarpus), or the tuft of hair.
Source: Webster's dictionaryThe tail is usually "banged" (cut straight across), usually to a length between the fetlock joint and lower hock. Source: Internet
The legs of the pony are wrapped with polo wraps from below the knee to the fetlock to prevent hurting. Source: Internet