Theobromine present in chocolate is poisonous to humans in sufficient quantities but usually doesn't affect us because we metabolize it very quickly. Domestic animals, including cats and dogs metabolize theobromine very slowly leading to poisoning leading to internal bleeding, heart attack and death. Apparently it's more hazardous to cats but they are unlikely to consume it in large quantities so it's rarely a problem. Generally the larger the dog the lower the risk but a given ration is about one one hundredth eg 20 kg dog / 20 g chocolate.
As stated previously theobromine content is much lower in white chocolate and highest in dark / cooking chocolate and Chocolate drops for dogs don't contain any at all.
In the good ol' USA it's being used as the base for a Coyote poison.
Not an old wives tale.