B-05 WHY DOES GOKU HAVE A TAIL?

As far as the DB anime series goes, it is a mystery why Goku has a tail. Later on, in DBZ, we find out that Goku is an alien, of the warrior race known as "Saiya." All Saiyajin have tails, which is one of the requirements for turning into a were-monkey. More on the subject of "Saiya" will be covered in question B-11.