Tenza

Tenza are a species of alien chameleons with powerful psychic abilities.

Life cycle
The Tenza are an alien species with similar behaviour patterns to a cuckoo bird. Shortly after birth, they drift in space looking for the nearest habitable world and a family that wants a child (but can not naturally achieve one). After locating a such a family, the Tenza infant would adapt itself to perfectly suit its enviroment and alter the surrogate parent's memory and the memories of those around them to believe the child was always part of the family.

After that the alien would live out its life as a member of that species, without consious realization of their true nature.

However, should a Tenza feel physicaly or emotianly threatened, their telepathic abilities would start to play havoc with their enviroment. Espesially if they feel they are no longer wanted by their surrogate parents. This was the case with George; who believed his parents wanted to get rid of him because they were struggling to help him with his phobias. This caused his powers to bring his phobias to life in a dollhouse that was governed by the Peg Dolls, who would terrorise him and abduct anyone who terrified him.

Everthing was put right when the Doctor was able to help George's father realise that despite what he was, their was still a paternal bond between father and son.