"cannon or cannon bone: The area between the knee or hock and the fetlock joint, sometimes called the "shin" of the horse, though technically it is the metacarpal III."
"fetlock: Sometimes called the "ankle" of the horse, though it is not the same skeletal structure as an ankle in humans. Known to anatomists as the metacarpophalangeal (front) or metatarsophalangeal (hind) joint; homologous to the "ball" of the foot or the metacarpophalangeal joints of the fingers in humans."
Currently, these are the worst of both worlds, to a layman reader. They include the old terms, without stating the reason for the misconception: that the hoof was a foot, therefore the fetlock was an ankle, etc. They include the new scientific terms, without explaining those or the new perception of the rear legs as having, to equate them to human ones, elongated feet and resting on a toe bone.