Can't remember the name of the story, but for some reason this one keeps popping into my mind for this thread.
Starts off with a guy taking his shoes out of the refrigerator (or putting them in, don't remember right now) and then he turns around and sees a horrible monster grinning at him. He freaks out and then looks again and the monster is still there but it's changed form.
The man starts to say how he thought this was over and then calms down and goes about his breakfast in the kitchen. The monster keeps changing forms and the man is now just getting annoyed claiming that this is all in his head and not real, even mocking the monster's form a couple times.
The man then gets angry saying he isn't scared of it and he's dealt with worse things. Then the thing speaks which now surprises the hell out of him because apparently it's never spoke before. The creature assures the man that he's real but the man regains his composure stating that he's still not real and that his own mind has just gotten worse.
The man and creature going back and forth over its existence for a moment until the man issues a challenge that if he's really real he should be able to pick something up. The man then puts both of his hands in a jar, stating that if his hands are in the jar then he can't pick up anything so the creature wont be able to either.
The creature suggests that he could turn off the lights instead then they could be in the dark. The man tells him to do whatever and to stop stalling. The creature goes to pick up a toaster but before it does so it states if the man is so sure that the creature is a figment of his imagination then perhaps the best way to prove he's real is to get rid of the man.
The creature then lunges at the man and rips his heart out.
The stands over the man's body laughing and then suddenly the creature begins to disappear causing it to scream out the big "Noooooo!"
Story ends with the man laying dead in his kitchen with his hands still in the jar.