Second Gorgon

The second Gorgon of three, Medusa and one that possessed an abbess, came to Earth around 1000 BC via a talisman, entering Greek mythology on the way. The first, Medusa, was slain by a Greek hero, leaving only two. However, the talisman was lost. Around the 1950s, the second Gorgon went looking for the talisman which had been found by Edgar and Bea Nelson-Stanley. The Gorgon managed to turn Bea to stone but was slain by her husband Edgar. Using the talisman Edgar was able to return Bea to flesh, leaving only one Gorgon to look for the talisman. (TV: Eye of the Gorgon)

Alternate timeline
In Andrea Yates' World, where the Trickster manipulated events so that Sarah Jane Smith died in 1964, the Gorgons never came to Earth. As the Trickster explained to Sarah Jane while she was in limbo, he had turned away all the foes she would have defeated, his intention being for her absence to allow Earth to be destroyed by the meteorite K67. (TV: Whatever Happened to Sarah Jane?)

Behind the scenes
While Eye of the Gorgon names Stheno and Euryale on screen as the other two siblings of Medusa, the story does not make clear which of the second and third Gorgon is which.