The note to the Somerset Wassail in the Oxford Book of Carols saya, "This Wassail was noted about twenty years ago [i.e. approx. 1908] from the Drayton wassailers in Somerset [Drayton is just south of Langport, seven or eight miles SW of Galstonbury] ... Sharp thought that the great dog of Langport was a reference to the Danes whose invasion of Langport is not yet forgotten in that town."

In fact, this Danish raid may be mere legend, as it seems that the Vikings never penetrated that far into the West Country. Their attempted invasion began on Christmas Day 877, when Guthrum's surprise attack on Chippenham drove Alfred into the marshes of west Somerset. Alfred set up a base at Athelney (the Island of the Nobles) a few miles west of Langport, and immediately began organising his counter-attack. In 878 he defeated Guthrum at Edington (the Anglo Saxon Chronicle identifies the Edington near the Westbury White Horse, although there is a theory that it was the Edington by the Polden Hills near Glastonbury). It was the resulting treaty between Alfred and Guthrum which divided England into the Anglo Saxon kingdom and the Danelaw.

I think the only Danish attack on the West Country was by the force which arrived at the mouth of the Parrett and was wiped out at Cannington. If they had got any further, they would have come up against Alfred himself at Athelney.

My theory about the girt dog is that the wassailers remembered another incident. There was this farmer who had a big dog, and one evening he took it down the pub, where the dog curled up by the fire and fell asleep. The dog had an exciting dream about chasing rabbits, and wagged his tail in his sleep, and his tail went into the fire ... It was probably the most memorable event since Alfred had come through there 1000 years earlier.