Time Vortex leeches were a species native to the Time Vortex.
The Eleventh Doctor believed them to be a myth from ancient Gallifreyan songs, until he found one clinging to the outer shell of the TARDIS. The effect of the leech was that time inside the TARDIS was running backwards in incremental jumps and would have led to the eventual explosion of the TARDIS. The Doctor prevented this outcome by transferring the leech to a black hole bomb that contained sufficient amount of quantum data to facilitate the leech's journey back in time. The Doctor hypothesised that it simply wanted to go back to something. (COMIC: Space in Dimension Relative and Time)
Gallifreyan songs described Time Vortex leeches as impossibly old creatures that lived in the Time Vortex, latched onto engines of space-time vessels and reversed them to take travellers back to their lost loves. (COMIC: Space in Dimension Relative and Time)