Almost all of the creatures in Middle Earth fear the evil Lord Sauron, and his dark powers that infest the lands, including the races of elves, men, dwarves, and hobbits alike. The unrest of the trees in Fangorn Forest, the vicious and loathsome spiders in Mirkwood, and the Crebain of Dunland, the flying crows who act as spies for Saruman the white, are all creatures who have been turned evil by the shadow that lingers in the cruel presence of the Lord of the Ring. But one particular breed of creature that is well known for its allegiance to the enemy of Middle Earth, and for its service to the 9 Ring-wraiths in particular, is the signature black horse upon which they ride.
Many other horses are shown to be terrified of the power the evil things in the world possess, for example the ponies who abandon Frodo and the other hobbits in the Barrow-Downs when set upon by a Barrow-wight. Even the bravest, most noble horses in the story, such as Shadowfax who is a great ally to Gandalf, and Bill the Pony who escorts the fellowship to the Mines of Moria, shy away from the treachery of foul beasts like Orcs and trolls. But many of the black horses who carry the wraiths are of a particular breed, created by Sauron himself to endure his will and carry out his bidding.
These horses are born in the very pits of despair behind the black gates, and raised under his total domination, to the point where they have no will of their own. They are a fearsome sight in the shire and pursue the four hobbits all the way to Weathertop, where their master stabs Frodo with a Morgul Blade. Interestingly, only 5 of the Nazgul are present at the time, because the other 4 are following Gandalf, who is trying to lead them away from the ring-bearer.
Aragorn manages to stay the wound using the Athelas plant of ancient herb lore to treat it, but still the horses and their riders follow the companions, who will later make up 5 of the 9 members of the fellowship, all the way to the Ford of Bruinen, on the edge of Rivendell. In the 2001 film adaptation, it is Arwen who calls the river to crash down upon the enemy as they pursue her and Frodo into its waters, but in the book, it is an ancient power cast by Elrond that causes the great waves, with an added flare of Gandalf who enchants them to look like riders on white horses in the sea-foam. The black horses of the Ring-wraiths are greatly affected by the elven allure of the place, and are described as being "filled with madness, and leaping forward in terror they bore their riders into the rushing flood."
Beyond this point, they are not seen again throughout the rest of the film adaptations. It is told in the books at the Council of Elrond that ‘3 of the black horses had been found at once drowned in the flooded Ford’ and that ‘the bodies of 5 more had been discovered on the rocks of the rapids below.’ Although the horses clearly didn’t survive the onslaught of waves, their cruel masters did, as they can be shapeless and abandon their physical forms in favor of a spirit-like appearance.
The Ring-wraiths were able to journey back to Mordor and regroup after reporting to Sauron that the Ring had made it safely to Rivendell. This is why the companions are not able to linger there safely for long and are forced to keep on with the journey towards mount doom where they can destroy it once and for all. The Wraiths themselves are seen again during the battle of the Pelennor Fields, riding the winged and screeching Fell-beasts that eventually kill King Theoden of Rohan before his niece Eowyn is able to cut off the creature’s head and defeat the Witch-king.
Many of the creatures infected with Sauron’s malice die during these great battles of Middle Earth, but those who survive are thought to flee up into the mountains and the hills, or to be taken to safe places, where it is hoped that they might be treated for their maladies, purged of the evil will of their former master, and returned once more to a life of peace away from war and greed. When Aragorn becomes the King of Gondor in the fourth age, and helps set up the Reunited Kingdom, he is able to assuage many of the peoples who sided with the enemy, including the Haradrim from the south, and the men of Dunland who helped to pillage the lands of Rohan whilst the king was under the poisonous influence of Grima Wormtongue.