Horizontal and vertical wall fissures.

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Horizontal fissures in the outside wall of the hoof are usually a result of a severe illness or metabolic disorder which affected hoof production from the coronary bane for a period of time. These fissures will grow out but they can be painful at times, corrective paring of the fissure to make it shallower often helps.
Horizontal fissures are often a result of either weak horn due to inherited or nutritional factors or they may be caused by a traumatic event e.g the foot becoming wedged somewhere.
Provided the horn has the capacity to regrow good quality, hard horn these defects will resolve themselves; however because of the slow growth rate of horn this takes a considerable time.