Walnuts — especially black walnuts and any mouldy nuts — can be seriously toxic to dogs.
Mouldy walnuts often grow tremorgenic mycotoxins that cause severe neurological signs. Black walnuts are also independently toxic.
Mouldy nuts can be colonised by Penicillium species that produce penitrem A and roquefortine, which inhibit inhibitory neurotransmitters in the brain and cause whole-body tremors and seizures. Black walnut hulls contain juglone, which is toxic to dogs (less so than to horses). Fat content also poses a pancreatitis risk.
Bronnen
Any mouldy walnut ingestion is an emergency. Fresh English walnuts in tiny amounts may not cause harm but are best avoided.