Yes, chocolate is toxic for horses. As for the dog and many other animals, this is due to théobromine contained in large quantities in cocoa.
The toxicity depends on the theobromine’s concentration of the chocolate and the weight of the animal, which is why intoxication is much more frequent in dogs. A dose of 100 to 250 mg / kg body weight is potentially fatal. For a dog of 10 kg it represents about 450 gr of milk chocolate or 60 gr of dark chocolate, so, for a 500kg weight horse it represents 3kg of dark chocolate!
Although it is difficult to reach this toxic dose in horses, it is recommended not to give chocolate to horses.