1) The large wooly mammoth featured on the children's television program, Sesame Street. He could only be seen by Big Bird
2) A person or thing that either exists only in one's mind or others have never met, therefore think only exists in one's mind.
A word that originated from the name of a character on Sesame Street, a snuffaluphagus is someone who can only be seen and interacted with by one other person - like an imaginary friend.
In series one of Scrubs the writers were unsure whether the Janitor was seen by all the characters or whether he was a snuffaluphagus seen only by J.D.