Well, there’s poisonous, toxic and indigestible. Various things can be one of those three to various other things; since mushrooms came first, you’ll find that most of them tend to be toxic to other organisms that haven’t evolved to consume them. Also, it’s good to differentiate between the mycelium and the fruit; often the mushroom fruits ARE poisonous to mammals because they propagate by airborne spores, so the poison evolved as a protection against being eaten before they could spore.

Other mushrooms depend on other features for their survival, from looking like a poisonous mushroom, to not fruiting, to making digestion by an animal part of their reproductive process.

There’s probably more variety in the mushroom kingdom than there is in the animal kingdom.