The origin of the word spaniel is described by the Oxford English Dictionary as coming from the Old French word espaigneul which meant "Spanish (dog)"; this in turn originated from the Latin Hispaniolus which simply means "Spanish".

In Edward, 2nd Duke of York's work The Master of Game, which was mostly a 15th-century translation of an earlier work by Gaston III of Foix-Béarn entitled Livre de chasse, spaniels are described as being from Spain as much as all Greyhounds are from England or Scotland. Sixteenth-century English physician John Caius wrote that the spaniels of the time were mostly white, marked with spots that are commonly red. He described a new variety to have come out of France, which were speckled all over with white and black, "which mingled colours incline to a marble blewe".
In the appendices added to the 1909 re-print of Caius' work, the editors suggested that the type of dogs may have been brought into the British Isles as early as 900 BC by a branch of the Celts moving from Spain into Cornwall and on into Wales, England and Ireland. Theories on the origin of the Welsh Springer Spaniel support this theory, as it is believed that the breed specifically is a direct descendant of the "Agassian hunting dog" described in the hunting poem Cynegetica attributed to Oppian of Apamea, which belonged to the Celtic tribes of Roman Britain:

There is a strong breed of hunting dog, small in size but no less worthy of great praise. These the wild tribes of Britons with their tattooed backs rear and call by the name of Agassian. Their size is like that of worthless and greedy domestic table dogs; squat, emaciated, shaggy, dull of eye, but endowed with feet armed with powerful claws and a mouth sharp with close-set venomous tearing teeth. It is by virtue of its nose, however, that the Agassian is most exalted, and for tracking it is the best there is; for it is very adept at discovering the tracks of things that walk upon the ground, and skilled too at marking the airborne scent.
Another theory of the origin of the spaniel is that the ancient Romans imported spaniels into Britannia by way of the trade routes to the Far East. Colonel David Hancock adds a belief that the sporting type of spaniel originated in China from the short-faced ancestors of dogs such as the Pekingese, Pug and Shih Tzu. The theory goes that these ancestors were introduced into Southern Europe and evolved into the small sporting spaniels of the period around AD 1300–1600. The issue of how a short-muzzled dog could evolve into a longer-muzzled dog is addressed by pointing to the evolution of the King Charles Spaniel into the Cavalier King Charles Spaniel in less than a century.

In assisting hunters, it is desirable that spaniels work within gun range, are steady to shot, and are able to mark the fall and retrieve shot game to hand with a soft mouth. A good nose is highly valued, as it is in most gun dog breeds. They are versatile hunters traditionally being used for upland game birds, but are equally adept at hunting rabbits, waterfowl, rats, and mice. Whether hunting in open fields, woodlands, farm lands—in briars, along fencerows or marshlands, a spaniel can get the job done.

On the basis of function and hunting style, the Fédération Cynologique Internationale (FCI) draws a distinction between Continental and Anglo-American spaniels. The FCI places Continental dogs of the spaniel type in the pointing group (Group 7, sect. 1.2) because they function more like setters which "freeze" and point to game. Breeds in this group include the Blue Picardy Spaniel, the French Spaniel, the Brittany, the Pont-Audemer Spaniel, and the Small Münsterländer. The FCI classifies most other dogs of the spaniel type as flushing or water dogs (Group 8, sections 2 and 3).

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