Hesiod gives the fullest surviving account of Pandora's creation and the opening of her jar.
The key narrative appears in Works and Days, a didactic poem traditionally dated to the 7th century BCE. Hesiod presents Pandora as part of Zeus's response to Prometheus's theft of fire, linking the story to explanations of toil, suffering and the human condition.
Hesiod also mentions Pandora in the Theogony, but the account there is briefer. Works and Days supplies the better-known sequence: the gods prepare Pandora, she receives the container, and its contents spread troubles among humanity.
Homer is another major early Greek poet, but the Pandora jar story is not principally a Homeric episode. This is why identifying Hesiod is more precise than simply answering “an ancient Greek poet.”