Do you mean "how hot must the air in the balloon be in order to lift a horse in the basket"? First, how heavy is the horse? There are, after all, a variety of possible sizes and weights for a horse, from a miniature horse to a Clydesdale! Once you have the weight of horse, basket, and all, you need to balloon to expand to a volume such that the weight of air displaced, minus the weight of hot air still in the balloon (you can use the "gas equation", PV= NRT to find that) is at least the weight to be lifted.
