Plain walnuts mainly carry a fat-related risk of stomach upset and pancreatitis, but moldy walnuts produce tremorgenic mycotoxins that can cause serious neurological signs like tremors, seizures and hyperthermia — never feed spoiled nuts. Black walnut in particular contains juglone in its bark and shavings; horses bedded on black walnut shavings can develop laminitis within as little as 8 hours of exposure.