Vis Sister

Vis Sisters are the bosses of The 7 Planes of Hehenna. They appear to be huge, fatter, versions of Gimps, and they also can spawn Hell Nibblers to assist them. You can't kill Vis Sisters directly, but you can break the floor under them, so that they will fall in the lava (but be careful not to fall there yourself).

Trivia

 * Vis Sisters were implemented in the Afterbirth+ DLC of The Binding of Isaac: Rebirth as bosses, albeit functioning entirely different and instead going by the name Sisters Vis.