ἑῷ

From LSJ

γνοίης ὅσσον ὄνων κρέσσονες ἡμίονοι → you know how much better are donkeys from mules

Source

French (Bailly abrégé)

dat. sg. de ἑός.

Russian (Dvoretsky)

ἑῷ: dat. sing. к ἑός.