History and specifics
The majority of the Zetarian people left their dying homeworld in the 14th century. About a hundred individuals were caught in a cataclysm which destroyed their physical bodies, leaving them as beings of non-corporeal energy. These beings then left the planet to seek out compatible humanoid bodies to possess.
In 2269, ten Zetarians attacked the Federation outpost on Memory Alpha, killing the population and inflicting significant computer damage. They then attempted to possess the body of USS Enterprise crewperson Mira Romaine, but were driven out and destroyed due to Zetarians' susceptibility to high pressure environments. (TOS episode & Star Trek 6 novelization: The Lights of Zetar)
Doctor Leonard McCoy once related the story of his encounter with the Zetarians with Doctor Jo Stern. She compared the dithparu discovered in the Well of Souls in 2336 to the Zetarians and the Organians. (TLE novel: Well of Souls)