It is character in Johnny, The Homicidal Maniac
comic book. Nailbunny is a little, dead bunny that Johnny has nailed to his wall. Nailbunny is the voice in Johnny's head that serves remotely as a conscience and tries to counter - effect Pillsbury Dough Boys
, Psycho Doughboy and Mr. Fuck, who manipulate Johnny into killing for evil, not fun.
Nny (Johnny) fed Nailbunny once and nailed him to the wall, but Nailbunny doesn't hate Nny, he just wants Nny to be happy.