A sark (or Sargh) was a type of animal lifeform, presumably native to Qo'noS, that were used as beasts of burden and as riding animals on many worlds colonized by the Klingon Empire. (TNG reference: The Starfleet Survival Guide, tlhIngan Hol reference: Klingon for the Galactic Traveler)
They bore some similarity to Terran horses, but with two straight, meter-long horns and flail-like tails.
Sarks may attack humanoids if provoked or very hungry. Furthermore, they were known to travel in herds, and may at times stampede. (TNG reference: The Starfleet Survival Guide)
Klingons would sometimes play games on sarkback, including one that involved copying mimicking the motions of another rider. (tlhIngan Hol reference: Klingon for the Galactic Traveler)