Tenza

The Tenza were a species of alien whose young assimilated themselves into other species.

Biology
Tenza had advanced, though subconscious, psychic powers, allowing them to live among other species. They used a psychic field and a perception filter to remove inconsistencies in their histories. One Tenza was capable of capturing people who frightened him, shrinking them to fit inside a dolls house and animating the peg dolls inside. He could also contact the Eleventh Doctor's psychic paper when he was millions of light years away, albeit subconsciously and in a fit of fear. (TV: Night Terrors)

Life cycle
Tenza were born in the millions in space, where they drifted until they found a habitable world. They would find a nest that they could live with and adopt them as their foster family. The Tenza infant would adapt itself to suit its environment and alter the memories of its surrogate parents and those around them. After that, the Tenza would live out its life as a member of the surrogate parents' species without conscious realisation of its true nature. Such Tenza had odd "tics" which drew attention; for example, Alex noted that George blinked too much when he was frightened, would stare off in to space at odd times and had never cried. If the Tenza felt threatened, its telepathic abilities played havoc with its environment to keep it safe. (TV: Night Terrors)

History
By 24 December 2002, Alex and Claire found that couldn't get pregnant, despite trying treatments such as IVF. By 2003, however, a Tenza came to Earth and assimilated itself into Alex and Claire's "nest", and they named him George. He was "born" in either January (TV: Night Terrors) or on 11 February. (PROSE: Night Terrors)

Alex and Claire raised him as their own, (TV: Night Terrors) but by the time George was seven and a quarter, he became deathly afraid of many things around him, and he and his mother came to put anything that scared him into his cupboard. (PROSE: My Special Book) His parents tried to help him, but eventually decided he needed professional help. George thought this would mean he would be taken away from them. This set off his psychic defences. His phobias came to life in a dolls house, which was inhabited by the peg dolls which terrified him, and abducted anyone who frightened him. He also contacted the Eleventh Doctor on his psychic paper. When the Doctor came to investigate, he realised that George was not human. This set off George's defences again, pulling both the Doctor and Alex into the doll house. When they were attacked by the peg dolls, the Doctor tried to get George to help them. This pulled George into the doll house, where he too was attacked by the peg dolls. Alex saw his son in danger and tried to save him. This proved to George that his fear of his parents sending him away was unfounded and everything returned to normal. The Doctor left Alex afterwards, telling him that George would grow up to be whatever his parents wanted him to be, though Doctor suggested he may return when George hit puberty. (TV: Night Terrors)