I think the poop monster is based 'the meatball' from the comic. The meatball is a curse on a certain family and only members from that family can see him. Eventually, the meatball appears before the family member and kills them by forcing meat - ie himself - down their throat. The family try gouging their eyes out, so that they never see him, but it doesn't work.
I have some of the Cat Eyed Boy manga. He lives in peoples' attics and causes severe misfortunes to befall those people.
As far as I can gather, his powers include: the ability to disguise himself as anyone (although the cat eyes always remain prominent); antiseptic tongue; acid spit; cat reflexes; ability to speak with animals.
He constantly gets beaten and impaled with sharp sticks...