The Grey Mare is the better horse. The woman is paramount. It is said that a man wished to buy a horse, but his wife took a fancy to a grey mare, and so pertinaciously insisted that the grey mare was the better horse, that the man was obliged to yield the point.
Macaulay says: “I suspect [the proverb] originated in the preference generally given to the grey mares of Flanders over the finest coach-horses of England.”
The French say, when the woman is paramount, C'est le mariage d'epervier (`Tis a hawk's marriage), because the female hawk is both larger and stronger than the male bird.
As long as we have eyes, or hands, or breath, We'll look, or write, or talk you all to death. Yield, or she-Pegasus will gain her course, And the grey mare will prove the better horse.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894