The Familiar were a race of barely sentient, water-dwelling, hive-minded creatures.
Characteristics
The Familiar were purely benevolent in nature, but impressionable; if they encountered the wrong person with enough influence, they would willingly follow him or her. (PROSE: Dead of Winter)
History
The Familiar popped up quite often throughout Earth's history — most often in anecdotes told by lonely travellers who had been "guided across a treacherous moor" by the supposed spirits of long-dead loved ones or friends that appeared to them. These "living ghosts" were in fact the Familiar, who read could read people's thoughts and use the information to create a near-perfect double of the one they loved most, solely for the altruistic purpose of helping them. After the Rory Williams got the Familiar to heal the Eleventh Doctor, his accumulated pain was too much for it and it was destroyed. However, a ghost of it remained and the Doctor used it to give Maria, one of its creations, back her mother. (PROSE: Dead of Winter)