Pluto & Scorpio
The Scorpion is the usual symbol associated with Scorpio. The most common tale of the scorpion comes from Greek mythology. A scorpion was sent by the goddess Hera, or sometimes it's said to be Gaia or even Apollo, to kill Orion, the hunter. People often correlate the reasoning behind Scorpio's associated symbol of the scorpion (aside from the obvious fact that they share a name), with the idea of the scorpion's sting, and how the sign of Scorpio sometimes can 'sting' another with cruelty.
I feel this misses the point of Scorpio entirely. A perhaps less well known but much more powerful symbol for Scorpio is the phoenix. A phoenix is a mythical bird found in various mythologies, including Egyptian and Greek, representing death and rebirth. The bird lives for hundreds of years and when it is ready to die, it ignites and burns to ashes, and the phoenix rises to live again. The sign of Scorpio embodies the process of the cycle of life, and the endurance and renewal of life. It digs deep, searches for truth, conquers fear and is often consumed by its ghosts in the struggle to overcome. The ancient planetary ruler of Scorpio was Mars, human will and life force. The modern ruler is Pluto.
Pluto (Roman) correlates with Hades (Greek) - they are God of the Underworld. In the Greek story, Hades was one of three brothers who ruled over the environments of mortals; Zeus/Jupiter ruled over the sky, Poseidon/Neptune ruled over the ocean, and Hades/Pluto ruled the underworld (a place that was also called Hades). Together they fought a war to overthrow the elder gods, in which Hades served a very useful role, donning his Helm of Darkness to become invisible and destroying all the elder gods' weapons. Hades was not death personified, but he ruled over the realm of the underworld. He was a just god but very feared, obviously, since meeting him usually meant you had died! One of the most well-known stories about Hades is the story of Persephone/Proserpine, also called Kore, which tells of how he gained his Queen.