Fat Sacks are a type of normal Fatties appearing in The Binding of Isaac: Rebirth. They look identical to Pale Fatties but with an open mouth and large, empty eye-sockets streaming with more blood.
They first appear in Chapter 3.
They behave very similar between the regular Fatty and Leapers. They leap off-screen and attempt to land on Isaac, shooting 6 projectiles outwards and leaving a red creep puddle behind once it lands.
They also can be found commonly in groups 1 to 3, and can be seen together with Pale Fatties.
Along with Pale Fatties, Fat Sacks will have a chance to turn into a Blubber upon death.