Sound more like an abcess and or infection pocket to me.
However, here is info on Selenium Toxicity:
Symptoms of Selenium Toxicity
Horses suffering from chronic selenium toxicity are said to have "alkali disease." The excess selenium causes the loss of mane and tail, the first indicator that your horse may be suffering. Many horse owners mistakenly label an innocent pasturemate as a tail-chewer when this occurs. However, upon further inspection, hair loss areas appear to have had a set of clippers run over them. A dull coat normally accompanies the hair loss.
Cracking, brittle hooves are another, more severe, sign of toxicity. A horse with this condition needs to see both a veterinarian and experienced farrier as soon as possible. Without treatment, the hoof may slough away from the coronet band and drop, exposing the sensitive laminae to the environment. This causes extreme pain and lameness for the horse. Even with treatment, the horse may be susceptible to abscesses until the new hoof capsule is regrown.
Finally, excessive selenium has been known to cause bone lesions and twisted legs in foals. Acute selenium toxicity, which is rare, may cause the death of the horse when massive amounts of selenium are ingested.
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