This topic might have a better name.
Sperovore, as it is spelt in A Life of Crime. However, it is spelled Speravore in the cast for The Diary of River Song: Series Two
Talk about it here.
Speravores were a squid-like species which fed off of potential. (AUDIO: A Life of Crime)
They were used by Golden Futures to feed off of the potential of their sleepers - as well as the Sixth Doctor - to produce energy for Project Elysium. (AUDIO: World Enough and Time)
They were defeated by River Song and the Sixth and Seventh Doctors. (AUDIO: The Eye of the Storm)