I had this idea for Polter the other day on his backstory.

OK. Here goes. He's human, and he happened to stumble into Eqquestria on accident. He tried telling people that he was a human from another realm, but not a single pony believed him. I mean, come on. If you told somepony you were from another dimension, would that be even remotely credible?
He was dubbed insane, and given a choice: the Insane Asylum, or a nearby ghost town of Hoofston. He chose the latter. And so now, he has spent about a month there, and has practically gone truly insane from lonliness. He even made himself a friend--litterally, as in from a broom, blanket, warning sign, pillow, rag, and hat--and has named "her" Nopony.
After all, Nopony believed him.
So, here he is, chasing the ghosts that terrify Eqquestria as his job. That how he got his cutie mark. That's what he does for a living.
I've even started this little fanfiction about his life, banised in a ghost town. Nopony gets possesed by one of these ghosts, and actually starts interacting with him. But he doesn't notice, since he has gone mad after all, and all the while he has believed she was actually a living pony and...
Celesta damn, this is complicated...
Well, if you want to read more, I've set up a FIMfiction account named after this character, PolterGeist. I'll try to post whatever I write there. I'll see you there!