From the Super Mario Wiki
“...I'm Screamy. I wonder what future is plotted for us by he who holds the compass... For I must deliver something...”
Screamy is one of Luigi's partners in Paper Mario: The Thousand-Year Door. Luigi met Screamy while looking for a piece of the Marvelous Compass in Rapturous Ruins. It is still unknown why or how Screamy joined up with Luigi as Luigi does not say and all Screamy says is "I must deliver something". It is unknown what species Screamy is; even Goombella doesn't know. Judging by his name and appearance, however, it's reasonable to assume he is some sort of ghost. However, since a lot of Luigi's adventures have to do with food, it might be that Screamy is a type of ice cream as well.
Brown Screamy-like creatures and their dark versions were originally going to appear as enemies, but were removed from the final version. These creatures may be prototype designs for Smorgs, as they are referred to as "moa" in the coding. This is further supported by the file dates, including the fact that the final Smorg design was labeled as "new_moa". 
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