A planet visited by James Kirk and Gary Mitchell before 2265, where Mitchell took a poisoned dart thrown by the rodent-like inhabitants that had been meant for Kirk.
Dimorus is never seen on screen but is recalled in the second pilot, Where No Man Has Gone Before. As Kirk agonises over what to do about his increasingly dangerous friend Gary Mitchell, he remembers an earlier landing on Dimorus during which the planet's rodent-like creatures attacked them with poisoned darts.
Mitchell deliberately stepped in front of a dart aimed at Kirk and very nearly died of the wound. The memory is one of several that make the decision to maroon Mitchell on Delta Vega, and finally to kill him, so painful for the captain.
The incident helps establish how long the two men had served together before the Enterprise's attempt to cross the galactic barrier in 2265.