gorgon
Who?
The birth child(ren) of the sea deities Ceto (sea monster), and her brother/husband Phorcys. In other cultures, it is told that the Gorgons were produced by Gaia, who needed more power to aid the Titans, because of the new Olympian deities.
What?
An immortal creature with the lower body of a serpent and the upper body of a woman. Snakes cover their heads, in place of regular hair. Their main power is the snakes covering their head, because if a mortal–being looks into the eyes of the snakes, that person will be turned to stone.
When?
Dated back to early Greek-Roman literature.
Story:
The Greek and the Romans believed that there were three Gorgons. Stheno, Euryale and of course, Medusa. They were believed to be living around the front entrance of the Underworld. Diodorus and Palaephatus believed that the Gorgons resided in the Gorgades (islands in the Aethiopian sea). A story involving the legend Perseus is where Medusa meets her demise.
“And at the Gorgon-seats arriv’d at last:
But as he journey’d, pensive he survey’d,
What wasteful havock dire Medusa made.
Here, stood still breathing statues, men before;
There, rampant lions seem’d in stone to roar.
Nor did he, yet affrighted, quit the field,
But in the mirror of his polish’d shield
Reflected saw Medusa slumbers take,
And not one serpent by good chance awake.
Then backward an unerring blow he sped,
And from her body lop’d at once her head.
The gore prolifick prov’d; with sudden force
Sprung Pegasus, and wing’d his airy course.“
References (see below):
Stolen Grace 
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