They show off their Poké Ball caps to lure prey, but very few Pokémon are fooled by this.
(From Bulbapedia)Amoonguss is a mushroom-like Pokémon with a large, light-colored Poké Ball designed cap with warts. It also has a huge gray, stem-like body, fierce looking eyes and a large pink mouth. Amoonguss wields two Poké Ball-like shields. It uses the Poké Ball-like patterns on its arms to lure in prey, though it doesn't work too well in most cases. Amoonguss is a carnivore which can be found mainly in long grass, as only their mushroom cap head protrudes from it, generally close to water.
(From Bulbapedia)It appears to be based on Amanita muscaria or mushrooms of the Agaricus genus, due to it being Poison-type. Its cap and the shield-like arms were designed to look like the front of a Poké Ball.
(From Bulbapedia)Amoonguss may be a combination of amanita (a genus of mushroom) and fungus or humongous (huge). Together with its pre-evolution, their names are a corruption of the phrase, "Fungus among us."