Monkshood ( Aconitum napellus ) can be deadly to animals and humans. This member of the buttercup family ( Ranunculacea ) contains aconitine, which is one of the most toxic plant compounds known. ( Ref. Encyclopdedia of Herbs and Their Uses, Deni Brown ) It is often called the Queen of poisons. In fact many common garden plants can be toxic to pets and/or people. For a list of potentially harmful plants checkout RHS harmful plants.