Though the reason given for using a horse is being their weight is 'equivelant to 5 men' (which is true; the avg.-sized horse weighs 1100lb., which would be equivelant to appx. 5 men), this episode was written/aired before space travel had become more well-known, and the reality of putting a horse in orbit - for any reason - would be highly prohibitive (due to their size, the required capsule-size, and fuel requirements, which would be exorbitant - that's why monkeys and Laika, the dog went).