Some people believe that conehead remains are a separate species or even extraterrestrials. While that is certainly something to consider, the practice of skull elongation is well documented.
About a month after a child was born, its head would be bound between two wooden planks… or strapped to a cradleboard… or attached to really whatever a particular culture decided would achieve their desired shaped. Then, it would be continually applied for a period of time spanning anywhere from six months to a few years; the amount of time just depended on the desired outcome and shape. It has been practiced for thousands of years.