Gorthaur

The name Sauron developed from the Quenya tongue as the name of the dreadful spirit that served Morgoth in the First Age, and became a Dark Lord in his own right during the Second and Third Ages. Though he was most widely known as Sauron, he had a different name in the Sindarin tongue used by the Grey-elves of Beleriand, who called him Gorthaur the Cruel. In fact the names Sauron and Gorthaur are etymologically related, and both descend ultimately from a root word thaw, meaning 'detestable' or 'abhorrent'. The Sindarin name Gorthaur added the intensifying prefix gor- meaning 'dread'.

We have little detail about the usage of the name Gorthaur beyond the fact that it was Sindarin in origin. Presumably this means that the native Sindarin speakers of Beleriand, most notably the Elves of Doriath, would have used the name between themselves, though we have no record of such specific usage. The name appears to have fallen out of use after the First Age: at least, by the time of the War of the Ring the name Sauron seems to have been universally used.