Well I never found SMK that much fun, but yeah, there was a type of poison mushroom.
See there are a couple types of poison mushrooms, there are my favorite, the purple ones with the skull on them, the mario kart ones, which were kind of a strange addition to the game, and in SSBM there is the posion mushroom which is the way it is so that you don't know what type of mushroom it is. Also there are some other variations in other less popular games.
Also, in Mario Kart you kind of have to name the items yourself, I just always called that a shrinking shroom. Plus it wasn't in any of the other games.