If in doubt, always call your vet or the Animal Poison Line immediately.
While shop-bought mushrooms (button, portobello, chestnut) are generally safe in small amounts, wild mushrooms are a different matter. Species such as Amanita phalloides (death cap) and Amanita muscaria (fly agaric) are deadly. As it is very difficult to identify mushrooms accurately, all wild mushrooms should be avoided.
Veterinary emergency if wild mushrooms were eaten. Try to identify or photograph the mushroom to help your vet.