EarthBound
The Magic Cake "vivid dream" was, in fact, Ness's dream
Poo is something like an imaginary friend or a dream character created in Ness's imagination and made real by the Magic Cake. Everything about him and Dalaam are particularly surreal and weird, even for EarthBound. He and his equipment all come out of nowhere and have no real lore. He operates based on what Ness thinks is cool or funny, and is likewise much like some combination of Ness himself and other tricks he probably wishes he could do; some of Paula's PSI, and Starstorm. Food doesn't restore very much of his health, because he's not a real person. Water restores a lot of his PP, because he actually has an unlimited supply of PSI as long as his cool power doesn't run dry. The sanctuary in Dalaam is Pink Cloud... which is what Magicant, one's own dream land, was made of in EarthBound Zero. The only time Poo is mentioned before he is met is when Talah Ramah, a prophetic monkey, says his name... but it can be inferred that he simply saw the future as it was actually going to happen, didn't predict it. He specifically says, in the same monologue, that everything is predetermined.
The previous is so because someone else was supposed to be the fourth hero and it didn't work out.
Porky maybe? Some believe Everdred was supposed to be playable. Apple Kid is the most helpful NPC in the game but maybe whatever prophesized Ness's adventure decided, last second, he needed more help fighting. Picky seems to be Ness's friend but is probably too young.
Porky is hot for Paula
He kidnaps her twice. Sure he wasn't directly responsible for it the second time but was probably involved. Maybe directly... the Department Store Spook takes her to the top floor, maybe Porky took her to the Monotoli building via helicopter.