While 'fox' simply refers to the animal, a cunning canine, 'foxy' is an adjective that describes someone (often a woman) as attractive or seductive in a clever, sly way. There's no direct biological link; 'foxy' borrows the fox's reputation for cleverness to imply a kind of attractive cunning. Using 'foxy' to describe a fox itself is technically incorrect, but the subtle association is understandable.