Horses and ponies are known for their long, flowing manes and tails and coats of glossy hair. However, anything that can grow hair can also go bald. But any time a horse's or a pony's hair starts to fall out is not normal. This is a symptom of an underlying illness or infestation that needs medical attention right away.
When a horse does not have enough to eat, the horse's body stops sending nutrients to hair, hoofs and skin to concentrate on keeping the inner organs going. The nutrients the horse needs to maintain hair, mane and tail are copper and vitamin A. Since the hooves are also made of the same material as the hair, keratin, they will also be in poor shape. Starving horses will also eat their own tails and hair or the hair of their pasture mates.
Any allergic reaction to a chemical in a shampoo, fly spray or even the laundry detergent a horse blanket was cleaned in can cause an allergic reaction leading to hair loss. A allergy to drugs like the insecticide ivermectin can sometimes lead to hair loss and allergic reaction. The hair loss can be in patches or all over the body.
Several kinds of external parasites can lead to itching and subsequent patchy hair loss on a horse or pony. This includes horse lice, leishmaniosis (a microscopic parasite originating in South America) and fleas. A heavy infestation of internal worms will also cause hair loss, particularly on the tail.
The two most common fungal infections that cause hair loss are ringworm and rain scald. Ringworm causes round hairless patches with crusty, often oozing centers and a red ring around the edge. Rain scald (or rain rot) usually happens on the horse's back, neck and hindquarters. They start off as a series of small hairless patches but progress to becoming itchy lesions in the skin, making life very uncomfortable for the horse.
Very rarely, some horses will lose all of their hair, manes and tails when they are very young foals. These horses generally have skin infections all of their lives and often can't be ridden or driven because their skin will not tolerate harness or a saddle.
If the horse only has small patches of hair loss where the harness or saddle rests, then chances are that tack is somehow causing so much irritation that the hair is falling out. Be sure the tack is clean and free of debris or rough edges. Girth galls and saddle sores need rest from all work to heal.