Strepto was the home planet of the alien Water Hags. Its land was mostly boggy. Some time during the Middle Ages, it was apparently lost by the Water Hags, who claimed it vanished into thin air. (TWN: Something in the Water)
Behind the scenes
The non-narrative source REF: Doctor Who: The Time Traveller's Almanac stated the Daleks had stolen Strepto, making it one of the 27 planets they needed to activate the reality bomb. This is contradicted by DW: The Stolen Earth, which counts 24 planets being taken from 2009 and the remaining three from other eras named by the Tenth Doctor and Donna as the Lost Moon of Poosh, Pyrovillia and Adipose 3, causing the Shadow Proclamation's map to put the planets in alignment.