Snorri Sturluson: The Prose Edda
13th century, Icelandic: numerous copies in Iceland, Copenhagen.
Loki turned himself into a mare and proceeded to drive the giant’s stallion mad with lust for three nights in succession.
'There is a story told about Odin’s horse Sleipnir,' said Quintin. 'The Norse god Odin who used to go about in disguise and often appeared as a one-eyed wanderer.'
'Yes, the principal Scandinavian god,' said Miranda. 'He offered to slain warriors the bliss of everlasting conflict, death and glorious return to his home in Valhalla. But also the one who sometimes appears from out of nowhere to guide someone, or to offer help, when a traveller in some story or other might come to a farm and get lodging for the night, receive some valuable advice or information and then leave in the morning – but then when they try to find the farm again, it is nowhere to be seen. Like in the saga of King Hrolf and the saga of Hord.'
'And like the spooky castle of the Fisher King in Arthurian legend,' replied Quintin. 'Perceval left King Arthur’s court and was trying to find this castle that he had stayed at only briefly a short while before, but it seemed to have disappeared. But to get back to Sleipnir: Odin went up and down the world tree Ygdrasill on Sleipnir, who was a horse with eight legs and which some think signified a coffin carried by four men.'
'So what's the story?' asked Miranda.
'The gods commissioned Valhalla from a giant who built it with the help of his stallion. The horse carried far more stone in a single load than the gods had reckoned he would, and they got nervous when the giant was about to meet his completion date and would need paying the ridiculous price he had asked for completing on time.
'Loki had been the one to draw up the contract and so he was the one chosen to try to sabotage the work. He turned himself into a mare and proceeded to drive the giant’s stallion mad with lust for three nights in succession. The stallion did no work for the giant and the deadline was missed.
'After the requisite period of time, however, Loki gave birth to a foal, Sleipnir, who grew to be Odin’s own horse, which he used to travel between the worlds.'