well theobromine and caffeine are toxic to birds and most chocolate contains both of them. They, like a lot of wild animals, cannot metabolize it. Many animals will handle this by simply passing it either by throwing up or the other way through the guts but other dogs are more susceptible to the damage it can do on their insides.

Strangely, with household dogs that are looked after, the damage is a lot more likely to be big because just like kids that grow up in sterile envrionments free of dirt and danger, they haven't built up the immunity or whatever that is necessary to fight these sorts of things.