A passage from ROTK describing the fell beast of the Witch King.  I think this is the most detailed description of it to be found:

"...it was a winged creature: if bird, then greater than all other birds, and it was naked, and neither quill nor feather did it bear, and its vast pinions were as webs of hide between horned fingers; and it stank.  A creature of an older world maybe it was, whose kind, lingering in the forgotten mountains cold beneath the Moon, outstayed their day, and in hideous eyrie  bred this last untimely brood, apt to evil.  And the Dark Lord took it, and nursed it with fell meats, until it grew beyond the measure of all other things that fly; and he gave it to his servant to be his steed."

VE

 
He can tie me up and blindfold me any day!

Varda's Poetry Corner: The Chaining of Melkor

O wretched day!  I shall rue it forever!
So the impossible now comes to pass:
Sunlight and shadow are blurring together;
I, the invincible, conquered at last!

Into the Void, chained and shackled, they send me,
Empty and vast are the depths of my cell.
Banished to where there are none to defend me;
Trapped by these walls that no power can fell.

Cold and alone, I am all but abandoned:
All ties are severed, all bridges are burned.
Darkness and silence my only companions;
Sentiments spoken are never returned.

Yet, from the shadows shall come their unmaking:
Hid in my song is the sum of all fears.
Darkness shall fall on the world in its breaking;
Rivers will flood in the rain of their tears.

Weapons of mine are not silenced by sheathing,
Seeds of despair have already been sown.
Shadow, an entity, living and breathing;
Malice and fear have a will of their own.

Foolish are those who now call me defeated-
Darkness shall once again drown out the sun!
Light, the usurper, will soon be unseated;
I am eternal. My will shall be done.

"Elbereth they name her, and they call upon her name out of the shadows of Middle-earth, and uplift it in song at the rising of the stars."