Tenza

The Tenza were a species of alien whose young were raised by another species.

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

Life Cycle
Tenza are born in space, where they drift until they find a habitable world. If they find a family that they can live in, then infiltrate it. The Tenza infant will adapt itself to perfectly to suit its environment and alter the surrogate parents' memories and the memories of those around them. After that, the Tenza will live out its life as a member of the surrogate parents' species without conscious realization of its true nature. Such Tenza will have some odd "tics" which drew attention; for example, Alex noted that George blinked too much when he was frightened or would stare off in to space at odd times. If the Tenza feels like it is threatened or likely to be removed, its telepathic abilities will play havoc with its environment. (DW: Night Terrors)

History
At some point after Claire and Alex had learned that they were unable to have children, a Tenza came to Earth and adopted them as its surrogate parents. The parents named him George and raised him as their own son. Because of the perception filter, they considered him an odd child but had no idea he was really an alien.

Before he was eight, George's phobias started to develop. His parents tried to help him, but eventually decided he needed professional help. George thought this would mean he would be taken away from his parents, and this set off his psychic defences. His phobias came to life in a doll house, which was governed by the Peg Dolls who would terrorize him and abduct anyone who terrified him. He also contacted the Eleventh Doctor on his psychic paper. When the Doctor came to investigate, he eventually realized that George was not human. This set off George's defences, 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, which caused him to be pulled into the doll house and attacked by the Peg Dolls. When Alex saw that his son was in danger and tried to save him, it 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 Alex should watch out for when George hit puberty. (DW: Night Terrors)