No info at all about this mask, but it appears now and then in several paintings, especially of the German-Venetian painters.
I even doubt if this character is coming from some non-Italian Carnival, and popped in there because it's funny looking.
The main image is coming from this Carnival masks and characters by Johann Georg Graevius which is the "Bacchanalia Venetorum" Engraving of a St Mark's Square depiction with all the stereotypes of Carnival masks of that period: Fatso can be found on the lower right side.
Most of characters in that Graevius composite image appear in other engravings and paintings too, information + communication was like this in those ancient times ...
But in the case of this Fatso character is worthwhile to give some attention, and I searched around and was able to find it here and there, especially related to Bulls Chases.
Maybe this wasn't a noble mask, that's why it didn't get any attention in the Bertelli's Carnival book, but it definitely deserves some attention.
The Fatso Guy characteristics:
If you get any information about this character, get in touch and please let me know.
I gave it the Fatso name, because he has a round belly, I found no reference whatsoever.