Riddle of Wings by Lferion

Winged am I, wind weaver     watching with keen eyes the world
Borne on my broad back     Fate battered brands from burning snatched
Crownéd I, I carry kings     Call, in might I come
Or Not; at narrow need     Near my knowing lord
All the air enfolds     Is his; I act at endings
Who am I?

Answer:

Thorondor, King of the Eagles, or more generally, one or more of the great Eagles.

References and commentary:

1. Eagles, especially Manwë's Eagles, fly high and have exceptionally keen sight
2. The Dwarves are rescued from the wolves and the burning trees, variously on backs and in talons. Sam and Frodo are rescued from the fiery destruction of Orodruin. Other rescues include Gandalf from Orthanc, and Maedhros & Fingon from the Thangorodrim.
3a. Thorondor is the king of the Eagles, thus either literally or symbolically crowned.
3b. Kings carried by Eagles include Maedhros (king of the Noldor), Fingon (who would be king of the Noldor later), Fingolfin (King of the Noldor, body borne to Gondolin after his battle with Morgoth), and Thorin Oakenshield, King Under the Mountain, twice.
3c. This is deliberately ambiguous -- if called/asked he/they might-maybe show up; if they show up, as to battle (end of the War of Wrath, Battle of Five Armies) they are ferocious and mighty. Both things are meant.
4a. Manwë is the Vala of Air, and can, if He wants to, perceive things anywhere there is air.
4b. Typically, Eagles show up at or near the end of things, or where an end will happen if they don't.