The Mini Mushroom (Community name Pikkusieni) found in Overgrown Cavern. They are the tiny fungus that is created from the material the green exploding mushrooms spawn.
They will generally ignore the player, but if you get too close, the Fungus will jump at you and violently explode, dealing a good chunk of damage. Killing them has the same result, so engaging at a distance is recommended.
They are not listed on the progress page at all.