The Mad Mushroom is an interesting fungoid organism found on the planet Gnosticus IV.
As a fungoid, it lacks any effective form of locomotion (and probably internal structures and brain). Therefore they are rather simple, spending all their time bouncing on the spot. They are however exceedingly toxic, and coming into contact with one is lethal. Their toxicity is their means of catching prey, upon which they feed.
They are immune to neural disrupter based weaponry, probably because they have no brain. The best strategy is to run under them as they jump; they will regularly perform an especially high jump after two small ones.
The official help states this:
Dangerous plants that hop and are deadly to the touch.
See also List of Keen 4 Creatures.
Appearance in Keen GBC
The Mushrooms are also seen on the Shikadi World in Keen GBC, where they behave almost exactly like their Keen 4 counterparts. They do however, have a larger cap and mouth, suggesting that they are a slightly different species. They can be killed from shots from Keen's Neural Ray Blaster unlike those in Keen 4.
The game manual describes them as the following:
Dangerous fungi that hop and are deadly to the touch. They jump higher every three bounces.