Before becoming known as Sauron, Tolkien’s dark lord was named Mairon.
Mairon was originally a Maia associated with Aulë, the Vala of craftsmanship. The name means “the Admirable” in Tolkien’s invented languages. He was powerful, orderly, and attracted to the idea of imposing perfect structure on the world, but he eventually became corrupted by Melkor, the first Dark Lord.
After joining Melkor, Mairon became Sauron, a name associated with the Abhorred. He served Melkor in the First Age and later became the principal enemy of the Free Peoples in the Second and Third Ages. He also used other names, including Gorthaur among the Sindar and Annatar, Lord of Gifts, while deceiving the Elves.
Melkor is Sauron’s master rather than an earlier name for him. Gorthaur and Annatar are later or alternative names, which makes “Mairon” the specific answer to the question about his original identity.