The typical horse neck is about 1/3 horse’s length, measured from poll to withers, with a length comparable to the length of the legs. A short neck neck is one that is less than 1/3 the length of the horse. Short necks are common, and seen in any breed.
A short neck is often quite flexible despite appearing thick and muscular, and the function and range is rarely altered. A short neck may be slightly less flexible at the poll, but the horse's maneuverability and agility are generally not affected too much.
In some horses, a short neck hinders the balancing ability of the horse, making it more prone to stumbling and clumsiness. A short neck also adds more weight on the forehand, reducing agility in some animals.
It does not shorten stride length, which has more to do with shoulder slope.
The horse may not excel at jumping high obstacles or galloping at high speeds, and may not be as handy at quick directional changes.