Toadstool
It has four types of attacks: it emits explosion mushroom spores that grow and explode after a few seconds, it jumps in place, spreading a shockwave, and it spits poisonous gas spores at the player's head, which explode after 3.5 seconds. The gas cloud deals 20 damage and accelerates the spoilage of food and spoilable items in the player's inventory. It can also grow spore mushrooms around itself. After burning, they leave behind the same poison.
The Toadstool's damage, attack speed, movement speed, and physical damage resistance increase each time the mushrooms on its back expand further (while the Toadstool itself takes reduced damage). Over time, the mushrooms become larger and lose their spots.
During combat, drops two mushroom skins. Upon death, drops another skin, a unique mushroom hat blueprint of a random color, and a blueprint for one of two types of mushroom lamps. Gains immunity to sleep and freeze if exposed to them frequently during combat.
Author: 17S
See also:
Binary and multi-element compounds
S.T.A.L.K.E.R.: Call of Pripyat - Radio Engineering
Portal 2 - Chapter 8: Itch (in two parts)