A horse with a "shark fin" has unusually high withers. They are most often seen on Thoroughbreds, and there may be some connection between high withers and speed - it may help the horse reach further forward with each stride. Shark fins can cause major problems with saddle fit (and trust me, never get on a horse with one bareback). Just as an obese horse may appear mutton withered, an underweight or very out of shape horse can appear to have higher withers (horses build muscle above their spine as a result of carrying the rider's weight).
The gelding in the foreground is slightly underweight (he's a rescue and very old) and at least appears to have shark fin withers.