Cerberus

I can see how Cerberus became envisioned.  Greyhounds have long and narrow bodies. They often stand abreast. If they look in different directions, and you see them for afar.  Κέρβερος was the multi-headed dog that guarded Hades, the hellhound. One of the labors of Heracles/Hercules was to capture and parade Cerberus.

The ancient Greek writers did not have a uniform description of the hound, or how many heads he had. Some descriptions also had snake heads included, usually on his back. The common description, that has come to us, is a black hound with three heads.

Peter Paul Rubens. Hercules and Cerberus. 1636. Madrid.