In Norse mythology, Svadilfari was a magical stallion, owned by a hrimthurs (rime giant) disguised as a human stonemason, who built the walls of Asgard and whose name is uncertain. Significance of the name is uncertain. Some sources translate it as “slave”, but Zoëga’s Old Icelandic dictionary makes no mention of it, suggesting instead the name might be related to svaðil-ferð :”disaster” and would mean “ill-fated”
After the walls surrounding Asgard had been reduced to rubble by the war between the Æsir and the Vanir, the gods were reluctant to take on the task of their reconstruction. The disguised hrimthurs arrived one day to present a proposal before the gods: He would build new walls to surround the city, stronger and higher than before. In return, he would take the Sun, the Moon, and the hand of the goddess Freyja in marriage.
The gods were outraged, but Loki counseled they consider the proposal, making the stonemason a counter-offer: He could have his price, but only if he was able to complete the walls in under six months (the time between the Winter and Summer Solstices), and with the aid of no man. If he could not meet both these conditions, he would receive nothing. The stonemason agreed, provided he could have the help of his stallion, Svadilfari.
The stonemason began construction by gathering massive slabs of stone and boulders, and loading them into a net hitched to Svadilfari, who then hauled them up the hill and helped to hoist them into place. When the gods saw how much labor had been accomplished in just one night, they were angered, because it was obvious that the mason was no mortal man, but a giant. However, this was not explicitly forbidden in their agreement, so they could do nothing.
As Summer Solstice approached and the wall neared completion, however, the prospect of it being finished on time became more and more likely. The gods began to worry that the stonemason would meet his deadline, and conferred upon the god Loki the responsibility of foiling him, since he was the one who’d talked them into accepting the giant’s proposal in the first place.
The next evening, Loki transformed himself into a beautiful mare and appeared before Svadilfari, who was overcome with desire and abandoned his master and his labors. Svadilfari pursued the mare all night, and the stonemason left his work to pursue his horse, which was enough to put him behind schedule, and thus lose his bargain with the gods. He became angry, and was revealed as a rime giant, not a man. The gods sent for Thor, who summarily dispatched the giant with his hammer Mjollnir.
Loki did not manage to escape the stallion, however, and reappeared only many months later, with the eight-legged horse Sleipnir, the result of his union with Svadilfari.
(Note: there is a popular misconception on the Internet claiming that the stonemason’s name was “Hrimthurs”, translating it as “Blast”. That translation is incorrect, and the name designates a particular tribe of giants, not an individual - see hrimthurs.)