Identification

• Sebaceous cysts in horses are hard, roundish lumps that grow up to 1 inch in diameter, according to "Horse Owner's Veterinary Manual." A sebaceous cyst in the "false nostril," or nasal diverticulum, is called an atheroma.
  
  

Treatment

• In horses, sebaceous cysts often become infected. Sebaceous cysts need to be removed surgically, especially if they are in the horse's nostrils and interfere with breathing.
  
  

Warning

• Do not squeeze a sebaceous cyst on a horse. If they open up and are not completely cleaned out, the horse is likely to get an infection. The inside pus, which usually looks like gray cream cheese, is made up of old, decaying body oil or sebum and will smell incredibly bad.