A Big Mushroom is an item introduced in Generation II. This item can be bought for 5000 and sold for 2500.
They are mainly held by wild Paras and Parasect 5% of the time excluding the Hoenn region games. Or 8% of the time in Generation II. These items don't do much, but only have an effect in Pokémon FireRed and LeafGreen on where the player needs to exchange two of these to the Move Relearner to allow his/her Pokémon to remember a move.