Which (individual) sages of Talmud were said to be traced back to King David? IIRC, R. Hillel was one (?). Are there others? Sources are welcome.
The Rashei Galusa in Bavel were descended from David, and we know that some of them were among the Amoraim. They include the earlier Rav Huna (a contemporary of R' Yehudah Hanassi, and possibly a teacher of Shmuel), Mar Ukva, and Rav Huna brei dR' Nosson. Also R' Nosson (the Av Beis Din in the time of Rabban Shimon ben Gamliel II) was a son of the Reish Galusa (see Horios 13b and the elucidation of it cited in note ה here).
R' Sherira Gaon also identifies the better-known Rav Huna (disciple and successor of Rav) as מבי נשיאה, from the family of the Nasi (Reish Galusa), which might also mark him as a descendant of David (although not necessarily: it could just mean that he was related to them by marriage, as was Rav Nachman).
The Wikipedia entry for Hillel says he also was a descendant of King David:
According to the "Iggeret of Rav Sherira Gaon" (a comprehensive history of the composition of the Talmud from the 10th century), Hillel descended from the Tribe of Benjamin on his father's side, and from the family of David on his mother's side.