The Simurgh is the god-bird or bird-king sought by the birds in the epic Persian allegory the Mantiq al-Tayr (The Parliament of Birds), a "monstrous bird . . . imagined as rational"¹. The birds never find him; we're informed that it gets a bit metaphysical at that point. Parts were translated by Edward Fitzgerald, of Rubáiyát of Omar Khayyám fame.

The Simurgh is verbal (I assume it speaks Farsi, though "Mantiq al-Tayr" looks kinda Arabic to me), has been said to have four wings, and is unspeakably ancient. I seem to recall that Iain M. Banks serves up a (not "the") simurgh in his novel Feersum Endjinn, though it might have been a lammergeier.

Some illustrators believe that the Simurgh is a peacock with the head of William S. Burroughs, as drawn by Ralph Steadman:

I'm not so sure. Ignore the illo, anyway; the text is "The Simurgh" by Jorge Luis Borges, and it's wonderful: "they cross seven valleys or seas, the next to last bearing the name Bewilderment, the last the name Annihilation". The Borgesian Or is one of the great delights of modern letters.

I'd node it if it weren't probably still under copyright. It's my main source for this writeup.