The oldest lore about Stonehenge dates back to Geoffrey of Monmouth's History of the Kings of Britain. Arthur wanted to commemorate a great victory over Hengist and asked Merlin his advice on the best way to do so and Merlin replied with "send for the Giant's Dance that is on Killare" in Ireland.
Arthur then purportedly went to Ireland and took the stones from there, brought them to England and erected them to stand "forever." Monmouth also makes great claims to the healing properties of Stonehenge and explains that any water poured over the stones has magical power to heal the sick. Monmouth as we well understand, was a fantastic storyteller and an abysmal scholar of history.
The great (if often curious) antiquarian William Stukeley made much of the lore surrounding Stonehenge and the inability for anyone to be able to count all the stones successfully. He claims he counted up to 140 and then exclaimed,
"Behold the solution of the mighty problem, the magical spell is broke, which has so long perplexed the vulgar. They think 'tis an ominous thing to count the true number of the stones and whoever does so, shall certainly die after it."
Another antiquarian recorded a belief that the Devil himself had somewhat to do with Stonehenge's placement. Aubrey in 1670 did write, "One of the great stones that lies down, on the west side, hath a cavity something resembling the print of a man's foot; concerning which the Shepherd's and Country people have a tradition (which many of them do steadfastly believe) that when Merlin conveyed these stones from Ireland by Magic, the Devil hit him in the heel with that stone, and so left the print there."
Aubrey and Stukeley were also the old antiquarians that told the Druid's must have built Stonehenge, since it was pre-Roman and yet was built by knowledgable men. Of course archaeology has disproved this theory with evidence that provides it was built far far before the Druids existed. But the notion has stuck in popular belief down the years anyway.