Gorgon
One of the three terrible sisters - usually the most famous one, Medusa - that resided
at the far West land of sunset in the Ocean, and whose legendary ugliness turned
men to stone when glanced at directly. 
Serpents for hair, boarlike teeth, a mad grin and glow in the eyes - all these are Gorgon's
features, which, so depicted, stared at owners of coins issued at more than two dozen Greek cities
ranging from Sicily to the Black Sea (albeit with less petrifying consequences).
Tales of her taming differ, but in the most common one
the hero Perseus, aiding his courage with Athena's mirror-like shield, manages to cut Medusa's
head off into a sack.
Since then, her facing image appears on this shield - under the name Gorgoneion - as a weapon, or charm of sorts, against other evils. Ironically, the modern Greek word "gorgona" follows possibly a later legend, and signifies quite the opposite of the petrifying ugliness of Medusa and her companions - the cold (and sometimes still quite torturous) beauty and grace of a mermaid. Or, perhaps it originates in the original appearance of Gorgon, who once was a beautiful centaur whom even Poseidon couldn't resist; their joint pleasure in Athena's temple was an act of sacrilege that the goddess was to punish severely for the non-divine of the two, with known consequences.
Lastly, it is interesting to note that this original form of Medusa can be found - crude and rare, but yet, of all places - on certain medieval Russian coins, her fame probably passed down with the written tales of the great Alexander.