Probably not gonna win, but meh. Mechanics: Mushcrabs would be a monster which, when a mushroom is destroyed, has a chance of spawning. They wouldn't be that strong, but would be able to come from any mushroom, with the colors changing to match the color of said mushroom. They would probably just be a nuisance, and they would come at any time of day. Idea: They are basically a mushroom with four claw-like legs coming from the base. The idea is that they are the mushrooms you have a chance of picking, and they get annoyed when you hit them. Another possibility is that they would awaken others from one or two of the surrounding mushrooms as well, making them very annoying. Attack: They would charge you and spike you with it's legs, but would also be able to release a spore cloud to buff you. The buffs would probably be something along the lines of drowsiness or paralysis. However, the cloud attack would also deal a small amount of damage to you, and each Mushcrab can only use this ability once, representing it's spore sacks being burst.
