What magic stallion helped a giant build the walls of Asgard?

The magic stallion that helped a giant build the walls of Asgard was Svaðilfari.

In Snorri Sturluson’s Prose Edda, an unnamed builder offers to fortify the gods’ home against giants and other enemies. The gods agree to pay him Freyja, the sun and the moon if he can finish the work in a single winter, provided he receives no human assistance. Loki persuades them to accept the deal, believing the builder will fail.

The builder’s stallion, Svaðilfari, makes the wager unexpectedly dangerous. The powerful horse hauls enormous stones and allows the wall to advance far faster than the gods expect. Near the deadline, the builder is almost finished, so Loki changes into a beautiful mare and lures Svaðilfari away from his work. The builder misses the deadline, reveals himself as a giant and is killed by Thor.

Loki later gives birth to Sleipnir, Odin’s eight-legged horse, after his encounter with Svaðilfari. Thus Svaðilfari is not Sleipnir: he is Sleipnir’s father and the builder’s indispensable work animal.