Butter is safe to eat
While butter is not toxic to dogs, it is also not something you want to be feeding your dog. Butter is high in fat and often contains added salt. Fatty foods contribute to obesity in dogs and cause other health issues such as diabetes and pancreatitis. Eating butter may also cause your dog to have loose stools.
Butter also contains lactose. If your dog is lactose intolerant, then eating butter will cause it to have gastrointestinal issues.