This is the third time the Pudgy Pig monster has appeared. He first featured in S1Ep6 "Food Fight" and reappeared briefly, as a mirage, in S1Ep28 "Island of Illusion, Part I." See more
Hey, you two. What do you think you're doing? You guys can't bring this pig in here.
[Rita's spell turns Norman into Pudgy Pig
It didn't say anything about that in the book.
Must have been something he ate.