E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Apollo received this aristocratic soubriquet from the following incident:Crinis, one of his priests, having neglected his official duties, Apollo sent against him a swarm of rats: but the priest, seeing the invaders coming, repented and obtained forgiveness of the god, who annihilated the swarms which he had sent with his fardarting arrows. For this redoubtable exploit the sun-god received the appellation of Apollo the Rat-killer. (Classic mythology.)