The Megans were the inhabitants of the planet Megas-Tu, in another universe where they were alone. Their universe allowed magic and wizardry, and the Megans were users of such forces.
The Megans were seemingly humanoid in appearance, indistinguishable from Earth Humans, although due to their magical abilities, they could take on many forms, such as those of spirits.
Millenia ago, in their search for companionship, they passed through the point between dimensions and eventually found themselves on Earth. Wherever they went, they became advisors to mankind, drawing on the power left behind in their own universe to make it work for Humans.
However, wherever they went, some Humans would attempt to use them to gain power and to serve their own greed and lust. When the Megans refused, the Humans turned against them and taught other Humans to fear and hate them. By the 17th century, those who survived settled in Salem, Massachusetts, attempting to live as "normal men" to avoid the accusations of being "devils, wizards, warlocks and evil sorcerers." Unfortunately, while living in Salem, several of the Megans made mistakes and used their powers. They were burned for doing so, as witches, during the Salem witch trials.
Those Megans who escaped the witch trials gathered their power and escaped to their homeworld, where they remained bitter and fearful of Humans.
In 2269, the USS Enterprise made contact with the Megans, but due to their distrust of Humans, they put the Enterprise crew in a recreation of a Salem witch trial. (TAS: "The Magicks of Megas-Tu")