'Nine [rings] he gave to Mortal Men, proud and great, and so ensnared them. Long ago they fell under the dominion of the One [ring], and they became Ringwraiths,...'
‘... the Black Riders are the Ringwraiths, the Nine Servants of the Lord of the Rings.'
‘What would they have done to me?' ... ‘What were the Riders trying to do?'
‘They tried to pierce your heart with a Morgul-knife which remains in the wound. If they had succeeded, you would have become like they are, only weaker and under their command. You would have became a wraith under the dominion of the Dark Lord; and he would have tormented you for trying to keep his Ring, if any greater torment were possible than being robbed of it and seeing it on his hand.'
‘But why could we all see their horses?'
‘Because they are real horses; just as the black robes are real robes that they wear to give shape to their nothingness when they have dealings with the living.'
‘Then why do these black horses endure such riders? All other animals are terrified when they draw near, even the elf-horse of Glorfindel. The dogs howl and the geese scream at them.'
‘Because these horses are born and bred to the service of the Dark Lord in Mordor. Not all his servants and chattels are wraiths! There are orcs and trolls, there are wargs and werewolves; and there have been and still are many Men, warriors and kings, that walk alive under the Sun, and yet are under his sway. And their number is growing daily.'
'I thought they were all destroyed in the flood'
'You cannot destroy Ringwraiths like that,' ... 'The power of their master is in them, and they stand or fall by him.
From The Fellowship of the Ring, by J.R.R. Tolkien