Hittites
A people the Bible mentions constantly and history had entirely forgotten — until their capital was dug up.
The Hittites are in the text from Genesis onward: Abraham buys a burial plot from them, Esau marries two of their women, and Uriah — the man David has killed — is one. For a long time no source outside the Bible knew of them, and their existence was treated as a problem. Then excavations in Anatolia uncovered a Hittite empire with its own archives. The relationship between that empire and the local "sons of Heth" Abraham deals with in Hebron is still debated; the point is that the name turned out to belong to somebody.
Disputed. The Anatolian Hittite empire is well attested. Whether the "sons of Heth" trading with Abraham in Hebron are the same people, a southern offshoot, or an unrelated group sharing a name is not settled.