A cockatrice is too strong to use as a regular familiar. It has high ability scores, decent skill ranks and a petrifying gazebite, making it too powerful for your average low level wizard. But if you just want a cockatrice for the flavor of having a cockatrice, you could use the rules for 'approximating familiars' as read here. You could use the stats of a chicken or peacock. Just imagine it's a runt or something.