Dimorus was a planet. It was the homeworld for a species of indigenous rodent-like creatures that had the ability to shoot deadly poison darts.
Prior to stardate 1312, Gary Mitchell and James T. Kirk visited this planet. During this visit, Mitchell intercepted one of these darts that was intended for his friend Kirk. Although he nearly died from the incident, he was able to make a full recovery. (TOS: "Where No Man Has Gone Before")
In 2376, the location of Dimorus was labeled on a star chart in astrometrics on USS Voyager in 2376. (VOY: "Child's Play", okudagram)