From WoWWiki

Horusaths are huge, powerful constructs of living stone, created by the Qiraji. The only known Horusath is Ossirian the Unscarred.

Warcraft lore now states that Ossirian was originally an Anubisath that was granted this new form after slaying the dragon Grakkarond.

Horusath looks like the Ancient Egyptian god Horus, who appeared as a man with the head of a falcon.