In light of Kobe Bryant going by Black Mamba, I think the rules of nicknames need to be clarified:
You cannot give yourself a nickname
Nicknames are what other people call you through affection, jest, or (occasionally) ridicule. You cannot ask for this privilege from others. If you want to go by something else, legally change your name; don’t call it a nickname.
You cannot ask for a nickname
Nicknames cannot be forced. They have to come through life experience, relationships, or achievements. This also means, of course, that you cannot assist in the formation of your nickname.
Nicknames must be honest or ironic
True nicknames are those that make sense at face value. Nicknames that say something interesting or colorful about the person are the best. If there’s nothing distinctive about you to get a nickname, either become more interesting or be content with your given name.
Use of the nickname is restricted
You may only use someone else’s nickname if you are part of the originating group that gave the nickname or were introduced to the person by their nickname or were asked to use it. Just like you can’t force a nickname to be created, you can’t force a nickname to be used.