(The). So called, according to fable, from Paeon, the physician who cured the wounds received by the gods in the Trojan war. The seeds were, at one time, worn round the neck as a charm against the powers of darkness. Virgil and Ovid speak of its sanative virtues. Others tell us Paeon was a chieftain who discovered the plant.
“Vetustissima inventu paeonia est, nomenque auctoris retinet, quam quidam pentorobon appellant, alii glycysiden.” —Pliny, xxv. 10.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Peony from Infoplease: