“Always behind, like a cow's tail.” “Tanquam coda vituli.” ( Petronius.)
What we have we prize not to the worth Whiles we enjoy it; but being lack'd and lost, Why, then we rack the value.
Shakespeare: Much Ado about Nothing, iv.1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cow's Tail from Fact Monster: