Some people have a nickname, and that's fine. Some canon characters have an inappropriate nickname, usually an embarrassingly fubsy diminutive given to them by their rabid fangirls. That's not so fine. Using an inappropriate nickname is a charge. In some cases, they become minis. In the Good Omens Official Fanfiction Summer School, they become mini-Hellpuppies: cute and fubsy and even more rabidly insane versions of mini-Hellhounds. In the Star Wars-verse, they become mini-rancor young.