I think the fighter earns it. Dont know whether they actually decided to name themselves, since 'Hitman' comes from HAtton's hard hits, or Arturo 'Thunder' is what we hear everytime a blow lands on Arturo's head. Lots of thunder going on when he was still alive. Marvin Hagler actually changed his legal name to 'Marvelous' because thats how he wanted to be called when he stepped in the ring. You wanna name your girl sweet pea or something?
usually u get a nickname in the gym. Unfortunately i train with a bunch of smartasses (cool guys though) so i often just get called ***** and gay, and payed out for being small.