Given this is a cairn associated with a possible dog or horse (See Note 4) it is worth looking at whether either might have been the original occupant of one of the cairns. It has to be said that it seems very unlikely.
This is not to say that the burial of animals in Britain isn't known from either the period of the legend or the cairn (See Note 6). Taking the probably age of the legend first: the burial of both horses and dogs is known from Iron Age and Roman Britain (burials are to be distinguished from deposition, though this is not always an easy distinction, even with fully articulated skeletons*).
Generally in the Iron Age dogs were buried within settlement areas, and therefore it is often hard to determine the reason for burial (some, for example, may be pets or the by-product of pelt removal*). Iron Age horse burials can warrant more physical evidence of ritualistic treatment (see, for examples, the burial at Nosterfield*) and the general importance of horses to Iron Age British societies is shown in their images on coins (see, for example, the Sedgeford hoard [Images and Info])
Romano-British dog burials were extensive and include, for example, some larger "dog cemeteries"*. In some cases there seems to have been the protective addition of dogs to the graves of the young* but possibly also an association with child sacrifice*.
Turning to the likely age of the cairns, both horse and dog burials are much rarer in the Bronze Age (following the trend for human burials). Horses and/or dogs were certainly important in the Bronze Age (see the White Horse (or Dog) of Uffington*, for example) but physical remains tend to be associated with human burials (see, for example, Horse at Poulton*, or late Neolithic Dog at Cuween Cairn [Info]).
It is difficult to say what or who was buried at Carn Gafallt. Although the frequent destruction of surface features where animal burials have been found means it is impossible to say that cairns weren't built over them, it is extremely unlikely a lone animal would have been treated in such a manner. That said, we can give some thin credence to the idea that an animal burial might be important enough to warrant a name in the Welsh landscape as we have the dubious evidence of Beddgelert ("the grave of Gelert"): traditionally said to be named after the grave of a early medieval dog (Details). Heck, if Alexander the Great can get away with it...