The English Hack is not a distinct breed but a horse type recognized in British equestrian tradition as the ideal pleasure riding horse: refined, elegant, well-mannered, and possessed of smooth, ground-covering gaits that make riding both comfortable and aesthetically pleasing. The word “hack” in British usage originally referred to any horse used for ordinary riding (as opposed to the hunter, bred for jumping cross-country), and over time it evolved to denote horses shown at hack classes at British county and national shows.
Show hacks are divided by height — small hacks (not exceeding 14.2 hh) and large hacks (14.2 to 15.3 hh) — and are judged primarily on presence, conformation, quality of movement, and manners. The ideal hack moves with freedom at trot, a flowing canter, and an extended walk; it stands quietly, moves off light contact, and is unmoved by show ring atmosphere. Thoroughbred or near-Thoroughbred blood predominates in show hacks, contributing the fine coat, clean limbs, and elastic paces associated with the type. The standard equine gaits page describes the movement qualities judges evaluate.
Outside the show ring, the term “hack” remains in common British use to mean any pleasure ride, particularly a leisurely trail ride — “going for a hack” — and the horse used for it. A well-schooled horse used primarily for trail and pleasure work may be described as a good hack regardless of breeding, provided it is safe, willing, and comfortable to ride.
See also the movement qualities judges evaluate at hack classes for why freedom of trot and flowing canter are weighted above scope, and the conformation points used in assessment for the physical framework applied when judging hack type horses in Britain.
Further Reading: The history of the hack horse type in British equestrian tradition, including show hack classification and judging criteria, is documented on Wikipedia’s Hack (horse) article.