The horse made a much better work animal than the dog because it could carry more weight than a dog. just by the fact it's so much bigger, so much stronger. It's strength was definitely the major reason why it was preferred to the dog, but the horse also adapts easily to its environment. Even if a horse eats more than a dog, because of its size, the benefits of using it for work are much bigger.
It's also much more calm and docile than most dogs, although some dog races are quite good at training and being used as assistance animal for the handicapped or blind people.
