via creepypasta:

In the show, Arnold is presented as an orphan who lives in his grandparents' boarding house. Some think that his grandparents are actually his real parents and because of a pregnancy so late in life, there were serious complications with Arnold's birth. He was born with hydrocephalus, a condition which causes a buildup of water in the brain, augmenting the size of the head—this accounts for Arnold's football-shaped head. Beyond that, conspiracy theorists also think that Arnold's friends are imaginary: he uses them to escape reality.