Hi Barbara,
The website I found Here!
it has this to say:
This Dartmoor song is one of the best known folksongs in England and can be found all over the world. It is thought to have been first heard in the early 1800's and the song first published either in 1880 by Mr W. Davies or in 1888 by Mr. W. F. Collier. However it was Sabine Baring Gould's book - Songs of the West that brought it to fame when it was published in 1890. Some people will say the tune originated in Somerset but we don't talk about them. The 5th Devon Volunteers used a more up tempo version as their theme tune and even after they were merged into the Devonshire Regiment the tune was heard as the men went into battle during the Boer Wars. Exeter City football club used to play the song before every match, that is until they hit a losing streak and thought the song may have something to do with their losses.What does the story line refer to? Clearly it is about a group of men who ask Tom Pearce if they can borrow his grey mare to get to Widdecombe Fair. The men agree that the horse will be returned by midday on Saturday by the latest. The designated time comes but the men and his horse don't appear. Tom Pearce then sets out in search of them. When he reaches the top of Widdecombe Hill he sees his horse making its 'will' after which it falls sick and dies. Poor old Tom then sits down and sheds a tear or two for his departed horse. Finally the song suggests that "when the wind blows cold on the moor of a night," the ghost of Tom Pearce's grey mare appears to the accompaniment of "skirling and groans," and the "rattle of bones." Strangely enough there has never been a single sighting of the ghostly grey mare. In the unmentionable Wessex version events are similar except the mare is a bay coloured and Tom Pearce goes to the top of Bunthill (which is thought to be Bonehill) and is told that there had been a "terrible spill," and that he found the "racketty crew.. " "strewed all over the shop." On the eve of Widecombe Fair the ghost of the horse appears in a cloud of blue light. In all possibility the men borrowed the mare to pull a gig as clearly it is impossible for eight men to ride a horse. Having had a busy time at the fair they got cydered up and overturned the gig on their way home which resulted in the death of the mare which roughly equates to the unmentionable Wessex version.
But is there any truth in the song? There was both a Bill Brewer and a Tom Pearce living in the northern moorland village of Sticklepath. The Pearce family owned a large mill in the village, outside which was said to have been the stable where the grey mare was kept. There is also a grave in the nearby village of Spreyton where a Thomas Cobley or Cobleigh was buried in 1844. This however is not the Tom Cobley, this person is said to be his great nephew. Thomas Cobleigh was 82 when he died reputedly having inherited his Great Uncle Tom's estate and who lived at Butsford. Tradition has it that the original Tom Cobley died in 1794 at Spreyton, nobody knows where his grave is. All the surnames are of true Devonshire stock, there are still plenty of Brewers, Stewers, Davys' Pearces, Whiddons, Hawks and Gurneys to be found today.
On Saturday, October the 19th 1850 the Exeter and Plymouth Gazette announced that on the following Friday (25th) a Free Fair would be held on the Green which adjoined the churchyard at Widecombe-in-the-Moor. It was expected that a large show of cattle and moorland sheep would be offered for sale. The following Saturday the same paper reported that "A cattle fair was held at Widecombe-in-the-Moor for the first time on Tuesday last." It was said to have been a busy affair with a large attendance of yeomen and gentlemen of the district where 736 sheep, 1,507 cattle, and 50 ponies were put under the hammer. The paper noted that due to its success the fair should be "permanently established," and so it came to pass. To this day the fair is probably one of the most famous events of the moor with visitors flocking to see the sheep, ponies, cattle, stalls, and events. The date has altered to the second Tuesday of September which just misses the busy holiday season. Some would say that the fair is congested enough without the additional numbers that holiday makers would bring but most will say that it does not clash with any other annual event.

I dont know how accurate this information is, but hope it helps answer your question!
Cheers, Lucy