The unwritten rule of tabletop Role Playing Games:
The Game/Dungeon Master has the right to veto anything any player says, he has the right to change any rule or make up his own, he need not explain why he choses to do these things. If players complain the GM may choose any of the following to do to the player; slap, call a dumbass, restrict snackage privileges and/or threaten injury to ingame character(be it through loss of xp, health, items or gold)
Player: Can I make my own species for my familiar
DM: Hells no! I'm gonna have to Rule 0 that one
DM: I swear I will cut you off of the mountain dew boy
Player: I'm sorry master.