Suetonius,
The Lives of the Twelve Caesars…
“ When Galba first declared against Nero, nothing gave him so much confidence of success, as a mule's happening at that time to have a foal. And whilst all others were shocked at the occurrence, as a most inauspicious prodigy, he alone regarded it as a most fortunate omen, calling to mind the sacrifice and saying of his grandfather. When he took upon him the manly habit, he dreamt that the goddess Fortune said to him, "I stand before your door weary; and unless I am speedily admitted, I shall fall into the hands of the first who comes to seize me." ”
