Mestor was a giant alien slug who served as the primary antagonist of the Doctor Who story, The Twin Dilemma.
When the Jacondan queen offended the sun god, he afflicted upon the planet a plague of giant alien slugs called Gastropods to cause a famine. When he saw what the Gastropods did to Jaconda, he repented and caused a drought which destroyed all of them, except for the leader, Mestor, who used his mental powers to enslave the Jacondans, as well as the Doctor's mentor, Azmael.
Mestor came up with a plan to spread his eggs all over the universe by causing a massive explosion from blowing up the Jacondan sun. To do this, he ordered Azmael (under the alias of Professor Edgeworth) to kidnap a pair of twin boys named Romulus and Remus so he could use their mathematical skills to move 2 of Jaconda's neighboring planets out of their orbit so the sun would explode.
However, the recently regenerated Doctor turned up and managed to convince the twins to get rid of their calculations. When he and Azmael went to confront Mestor, Azmael was possessed by Mestor, but the Doctor destroyed Mestor's body using a beaker of acid he found in the laboratory while Azmael sacrificed himself by triggering his last regeneration to kill Mestor.