Splayed feet, also described as splay-footed or toe-out, is a conformation fault in which the front hooves turn outward from the vertical axis of the limb, with the heels pointing inward. The deviation can originate at the knee, the medial joint stress from outward rotation, or the hoof itself. When the rotation is present from the knee down, the entire lower column swings outward; when it originates at the hoof capsule only, the rotation is more limited and the structural impact is less severe.
The primary consequence of splayed feet is abnormal rotational stress on the lower limb joints, coffin joint, pastern, and fetlock, as the foot breaks over the inner toe rather than centrally. This predisposes the horse to medial (inside) joint wear, ringbone, and windpuffs. The outward swing of the foot during the flight phase of the stride also increases the risk of paddling (an outward arc) and interfering (striking the opposite leg), both of which are faults visible at the trot.
Mild splay can be partially managed by corrective corrective trimming for straighter breakover: trimming to encourage a straighter breakover, but structural deviation at the knee or higher cannot be corrected by shoeing. In young foals, some degree of toe-out is normal and often self-corrects as the limb strengthens; in a grown horse, the deviation is permanent. When evaluating a horse for purchase, note whether the deviation is symmetric (present in both front feet to the same degree) and whether the associated joints show signs of existing wear or swelling.
Further Reading: The biomechanics of limb deviation and toe-out stance are covered in the Wikipedia article on valgus deformity. For the full context of structural evaluation, see the Wikipedia overview of horse conformation.