Mystery Theme VI: Topic Occluded by KlionskyG

Question 9

Joseph Louis Lagrange proved Bachet's conjecture, which states that every natural number can be represented as the sum of four integer squares. But Bachet didn't come up with the problem; he merely translated the much earlier Arithmetica into Latin in 1621.

Name the Greek mathematician who wrote this earlier work (which also led to a better-known conjecture about which I've written a truly marvelous question that this text box is too narrow to contain).

Diophantus (conjecture proved 1770, exact date unknown)