For some reason, this Pokémon resembles a Poké Ball. They release poison spores to repel those who try to catch them.
(From Bulbapedia)Foongus is a white mushroom-like Pokémon with a cap that looks quite similar to a Poké Ball. It has stubby, digit-less arms, a large pink mouth, and pinprick-like dark eyes. It uses the Poké Ball-like pattern on its head to lure people and Pokémon in grassy areas, then releases a cloud of poisonous spores to defend itself. It moves by either hopping or turning on its side and rolling on its cap.
(From Bulbapedia)It appears to be based on Amanita muscaria or mushrooms of the Agaricus genus, due to it being Poison-type. The cap is designed to look like the front of a Poké Ball.
(From Bulbapedia)Foongus is a corruption of fungus and may be a combination of fool (to deceive, referencing its use of its Poké Ball-like cap to attract unsuspecting prey) and fungus. Together with its evolution, their names are a corruption of the phrase, "Fungus among us."