Cecrops was an individual from the Collective of Mulch who protested against the time travelling of the Slitheen, believing that it would damage humanity's development. Cecrops was partially humanoid, with a human-like upper body and a snake-like tail.
Cecrops was estranged from his family, who thought he was too rebellious. He snuck onto one of the Slitheen's travel packages and went to Earth in 1500 BC. Though passionate about his cause, he had nothing to go on. He tried to befriend June to get her opinion as a native (not knowing she was also a time traveller). When she and the Tenth Doctor tried stop the Slitheen, Cecrops tried to help. When the situation was resolved and the other tourists returned to their own time, Cecrops decided to stay in the past so the Slitheen would be blamed for his "death" and be bankrupted. He stayed with the humans, helped build Athens and eventually was immortalised as the mythical King Cecrops. (PROSE: The Slitheen Excursion)