Cerberus or "The Hound of Hades" was the three headed dog who guarded the Gates of Hell between the living world and the Underworld. His job was to prevent the dead from escaping back into the living world. Cerberus was Hades ever faithful companion. They often rode around the Underworld together on Hades' golden chariot.
One of the most famous stories about Cerberus is the one where Eurytheus, king of the Tiryns tasked Hercules with the job of fetchching Cerberus from out of the Underworld. Cerberus was then to be brought to the king and shown off to his people. This task was the last part of the twelve labours that were imposed on Hercules. Eurytheus (and Hera who Eurytheus was in league with) actually believed these tasks to be impossible. The tasks were designed to kill Hercules. But, Hercules was not stupid.
Hercules asked Hades if he could borrow his beloved dog, Cerberus, to take to Eurytheus. Hades said he would allow it if Hercules "mastered him [Cerberus] without the use of the weapons which he carried". There are a couple of different versions of the tale regarding Cerberus's capture. One tale says Hercules gently managed to get his arms around Cerberus's three heads squeezed his neck until Cerberus finally submitted. Another story tells that Persephone gave Hercules Cerberus in chains.
When Cerberus was brought up from the depths of the underworld the sunlight shocked and upset him. The sun was too bright for the creature of darkness and it made him sick. Legend has it that his vomit made the Aconite (Monkshood) plants that it touched poisonus. Cerberus was enraged by the sunlight and the previously submissive creature struggled and fought against Hercules for bringing him out of the darkness. Hercules and Theseus were both forced to drag the kicking, fighting and screaming Cerberus out into the light.
Hercules paraded Cerberus in front of King Eurytheus, as instructed. Then Hercules took Cerberus back to the Underworld. Another version of the stories suggests Cerberus escaped from King Eurytheus and made his own way back to his master Hades.
According to Hesiod, Cerberus was the "brazen-voiced hound of Hades", who is obsequious about those that enter the house of Hades, but eats those who try to leave. It is also said by, Stesichorus that Cerberus was called the hound of Hades because anyone bitten by him died immediately. Euphorion of Chalcis (3rd century BC) describes Cerberus as having multiple snake tails, and eyes that flashed, like sparks from a blacksmith's forge, or the volcaninc Mount Etna.
Whatever the Greek myths tell us Cerberus was a dog who represents loyalty, love, fidelity, intelligence, protection, resourcefulness, and finally, cooperation and community. Similar traits to us, so, he was a good choice for a symbol for us.