Mucous or excessive watery drainage from both nostrils is a very common sign of bacterial or viral upper respiratory tract infection. Upper respiratory infection is usually highly contagious, so your first consideration may be minimizing spread of disease to other horses. Wash your hands with antiseptic soap before touching other horses. Do not share tack or equipment with other horses, until your vet has seen the horse and determined the nature of the problem.

Two sided nasal discharge is also seen in respiratory diseases originating in the lower respiratory tract -windpipe (trachea) or lungs - as opposed to the sinuses and nasal passages in which there is usually discharge from only one nostril.

Small amounts of clear discharge from equine nostrils is considered a normal finding. White discharge is commonly associated with viral or allergic processes. Yellow discharge tends to be associated with bacterial infection. Green nasal discharge can indicate feed material within it.

A distressed horse with profuse green, watery discharge (containing feed material) from both nostrils and a deep cough likely has choke (esophageal feed obstruction). In very rare cases, intestinal obstruction can cause backup of fluid that ultimately comes out the nostrils.