The Starchild skull is a real existing object and target of scientific studies. It is very unusual and abnormal human-like skull which was found in Mexico. Its origin and nature are contested by scientists and paranormal enthusiasts. The volume of the interior, where the brain lies is about 1600 cubic centimetres, which is 400 cc more than the brain's volume of an adult human with comparable size. There is an overload of collagen in it, much more than it is usual for the human bones and also there are no frontal sinuses. The carbon 14 tests gave a result of 900 years ± 40 years since death. Also it have other features unknown in others humans or animal species. There are not known diseases or causes that can cause similar shape and structure of a human cranium.
Sources: Wikipedia, Starchild project,