Short answers, pulled from the story.
Scholars have struggled for centuries to pin down the true origin of the name Cerberus. Linguist Daniel Ogden describes attempts to establish an Indo-European etymology as not yet successful. Some researchers point to a Sanskrit word sarvarā, used as an epithet for one of Yama's dogs, and trace it back to a Proto-Indo-European root meaning spotted.
Ancient writers varied wildly on how many heads Cerberus possessed. Hesiod's Theogony, dating to the eighth or seventh century BC, describes the creature as having fifty heads. Pindar, who lived between 522 and 443 BC, gave him one hundred heads. Later writers almost universally settled on three heads.
Homer first mentions the task when he says Heracles was sent by Eurystheus, king of Tiryns, to fetch the hound of Hades. Hermes and Athena served as guides during the journey. Euripides states that Heracles entered the underworld at Tainaron, the most famous Greek entrance to the realm of the dead.
The earliest depictions date from the beginning of the sixth century BC. A lost Corinthian cup from Argos shows a single canine head with snakes rising from its body. Another relief pithos fragment from Crete appears to show a single lion-headed Cerberus.
Medieval commentators reinterpreted Cerberus as a symbol of earthly corruption. Servius derived the name from creoboros, flesh-devouring, and held that Cerberus represented the corpse-consuming earth. Fulgentius allegorized the three heads as representing nature, cause, and accident.