
InNorse mythology,Svaðilfari is astallion that fathered the eight-legged horseSleipnir withLoki (in the form of amare). Svaðilfari was owned by the disguised and unnamedjötunn who built the walls ofAsgard.
Svaðilfari in Old Norse translates as "the one making an unlucky journey" or "unlucky traveler".[1][2]

Völuspá hin skamma, contained withinHyndluljóð, states that Svaðilfari fathered Sleipnir with Loki:[1]
| Old Norse text[3] | Bellows translation[4] |
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In chapter 42 of theProse Edda bookGylfaginning,High tells a story set in the early days and after the gods had establishedMidgard and builtValhöll about an unnamed builder who offered to build a fortification for the gods to protect them frombergrisar andhrímþursar. UnderLoki's advice, it was agreed that if the work was completed in one winter with the help of his horse Svaðilfari, the builder would be givenFreyja, thesun, and themoon. With Svaðilfari's help, the builder made fast progress on the wall, and three days before the deadline of summer, the builder was nearly done.[5][6]
Because of his role in the making of the wager, the gods declared that Loki would face severe repercussions if he did not think of a way to cause the builder fail to complete his task. That night, the builder drove out to fetch stones with Svaðilfari, and out from the woods, into the clearing, ran a beautiful mare who was, in fact, Loki in disguise. Svaðilfari saw the mare and became frantic, chasing her into the woods. As the two horses ran around all night, the building work could not be finished in time and realising this, the builder went into a rage (Old Norse:jötunmóð), revealing he was abergrisi.[5][6]
When the Æsir realised the builder's identity, they disregarded their previous oaths with the builder and called forThor who killed the builder withMjöllnir. Due to his night with Svaðilfari, Loki became pregnant and later gave birth to the eight legged horseSleipnir.[5][6]