Sour Grape

From KeenWiki

Sour Grapes Inhabit Grape Grove and a few other places in Tuberia; they appear harmless, and are while hanging from the ceiling (Or floating up to it.) but whenever they detect movement, they fall to earth, hoping to squash and consume any hapless prey that is beneath. They are tough and bouncy, being able to survive huge drops, and contain a ballast gas they can use to float after they have landed. They cannot be killed. It seems that these enemies are based on an enemy from the "Mario Brothers" series, the "Thwomp". Thwomps are giant spiky stones that plummet when the player is under it, and rises after a short time.