The aim of this report is to present a clinical case of an additional tooth (dens suplementarius) in a 13-year-old thoroughbred mare currently used for pleasure riding. She was referred for specific dental evaluation because of recurrent nasal discharge that started to change from mucous to mucopurulent. The first attempt to remove the additional tooth was performed on the standing horse, but was unsuccessful because of the location of the problem. Surgical procedures under general anaesthesia, that is, trepanation of the nasofrontal sinus and “sinus flap,” succeeded. The mare was under antimicrobial therapy for 7 days after the surgery and received NSAID drugs for 2 more days. Medical dressing was removed 2 days after the surgery, and sinus flush with Foleys catheter was performed during 8 days after removing the dressing. The mare returned to previous work soon after the surgical procedure. Dental problems are among the most common in field veterinary practice, but supernumerary teeth occur in only a small percentage of horses. Cases of such tooth abnormalities are usually detected in the early or middle age of an animal during a regular dental examination or when the situation starts to create clinical problems. A suspicion of an additional tooth should always be confirmed by imaging diagnostics. In most cases, radiological evaluation is enough to confirm the diagnosis and asses the situation before selecting a specific medical solution.