Conceptual Development: The name ᴱN. Thû was the earliest name of Sauron after the character transitioned into his later conception as the Lord of Werewolves, first appearing in the Lays of Beleriand from the 1920s (LB/16, 146). The name N. Thû appeared in Silmarillion drafts from the 1930s and also in the Etymologies as a derivative of the root √THUS “stench” (LR/29, Ety/THUS) but it was gradually replaced by his Quenya name ᴹQ. Sauron (SM/120, LR/283). The notes mentioned above seems to be a late remnant of his earlier name; Thû did not appear in the Silmarillion revisions from the 1950s-60s.
Elsewhere in “Words, Phrases and Passages from The Lord of the Rings” from the 1950s-60s, Tolkien considered using Thû as a name of Manwë from the root √THŪ “blow” (PE17/124), but this seems to have been a transient idea.