The Lightning Thief
by Rick Riordan
Such a cute little puppy. Cerberus guards the entrance to the underworld. He's a ginormous doggy with three heads. His job is to sniff out the living who might be trying to sneak into the Underworld.
I'd always imagined Cerberus as a big black mastiff. But he was obviously a purebred Rottweiler, except of course that he was twice the size of a woolly mammoth, mostly invisible, and had three heads. (18.121)