Nasal polyps are soft, painless, noncancerous, inflammatory growths that can affect the horse's upper respiratory system. They occur most often unilaterally as singular or multiple masses in the nasal cavity of the horse. Onset of signs can be insidious and worsen over a prolonged period of time. Nasal polyps are thought to form due to a variety of possible mechanisms which include allergy, enzyme, mechanical obstruction, epithelial rupture, and abnormal transepithelial ion transport. If nasal polyps obstruct sinus drainage, subsequent infection can cause more stasis and swelling, which end up causing a self-perpetuating cycle.

There are several conditions that are associated with nasal polyps which include allergies, sinusitis, rhinitis, aspirin intolerance, asthma, cystic fibrosis, primary ciliary dyskinesia, immunodeficiency, and churg-strauss syndrome. The severity of nasal polyps identified during endoscopy are evaluated on a scale from 0-3.