By the time of the main narrative of "The Lord of the Rings", Sauron had already learned from Gollum that a hobbit, Bilbo Baggins of the Shire, had acquired the One Ring. Sauron entrusted its recovery to the Nazgûl. They reappeared "west of the River", riding black horses that were bred or trained in Mordor to endure their terror. They learned that the Ring had passed to Bilbo's heir, Frodo, and hunted him and his companions across the Shire; the hobbits heard what seemed to be snuffling, and sometimes saw them crawling or crouching. The hobbits escaped, via Tom Bombadil's realm of the Old Forest where they were not pursued, to Bree. A Ranger of the North, Aragorn, arrived ahead of them and for some days led the hobbits on paths not closely followed by the Ringwraiths.