Pecans can cause stomach upset, pancreatitis, and (if mouldy) severe neurological signs.
Like walnuts, pecans are vulnerable to mould that produces tremor-inducing toxins. Their high fat content also strains the pancreas.
Pecans contain juglone (in lower concentrations than black walnuts) and are highly susceptible to colonisation by tremorgenic mycotoxin-producing moulds. The 70% fat content makes them a known pancreatitis trigger in dogs.
Surse
A pecan or two is unlikely to cause symptoms in a medium dog, but mouldy pecans and large amounts both warrant veterinary attention.