Fan Nickname/Newspaper Comics

"Blandthony" or "Blanthony", and "Pornstache" are two common nicknames for Anthony from For Better or For Worse in reference to his milquetoast personality in comparison to Liz's other suitors and his distinctive mustache, respectively.
In Dilbert, a talking cat was un-named for his first several appearances. Fans dubbed him "Catbert" (after "Dogbert" and "Ratbert") and Scott Adams adopted the name officially later on. So here we have a situation where the fans actually named the character outright.
- Deeper than that. The one off character was so popular immediately that not only did the fans name him, but he became a reoccuring character.
The Comics Curmudgeon has dubbed the Anthropomorphic Personification of death from Funky Winkerbean "Masky McDeath". It caught on among the commentators.