"Null" is not the monster name. Null a term on game creation that means a sprite/asset without name.
Also, the game's entity list lists something called a Null or Nulls, which has 30 HP, is listed right after Begotten, and deals half a heart of contact damage, and is considered a champion. I assume that's them. --Mrdiamonds (talk) 16:33, 3 December 2014 (UTC)
I just encountered Greed Gapers in a regular room, filled with dead shopkeepers and coins. So they appear not only in treasure rooms now.