Marcos XII

From Memory Alpha, the free Star Trek reference.

Marcos XII (or Marcus XII) is a planet in or near Federation space. It was the first planet which the evil angel-being Gorgan decided to conquer once he was set loose. Gorgan used the children of the Starnes Exploration Party to capture the Constitution-class starship USS Enterprise and was using the ship to go to there. Tommy Starnes had relatives on Marcos XII. (TOS: "And the Children Shall Lead")

Although this planet was outside Enterprise's normal patrol area, it is likely to be near Starbase 4 and the planet Triacus. It was also mentioned as a small note on a very large star chart graphic created for TNG: "Conspiracy". It was the image behind the chair in the room where Dexter Remmick was killed. It appears to be located within an outlined block, which may indicate that it was surrounded or was even a part of Federation territory at the time.