The King of the Eagles (S/110), his name is a combination of thoron “eagle” and the suffix -dor “king” (SA/thoron, Let/427).
Conceptual Development: In the earliest Lost Tales, this character was named Thorndor “King of Eagles” (LT2/192), a form that also appeared in Silmarillion drafts from the 1930s (SM/34, 102; LR/126). The form Thorondor first appeared as a late change in the Lays of Beleriand from the 1920s (LB/292), and this new form gradually supplanted the old one in the Silmarillion drafts (LR/145, 256). N. Thorondor was the only form to appear in The Etymologies, where it was translated “King of Eagles” and given the derivation described above (Ety/THOR).
thoron (“eagle”) + taur (“king”)