When we talk about horses in space we must qualify that by saying that we do not mean horses aboard space ship or space stations, cantering about. Horses are large animals and they take up a lot of space; and horses are not the brightest animals, intellectually speaking. Already the transport of horses terrestrially is expensive and time consuming; as exampled by thoroughbred race horses travelling to international events like the Melbourne Cup in Australia. No, when we refer to horses in space being more probable than humanoids we are talking about horse like creatures already established on their own planets in outer space.
Where the atmosphere may be thinner and so having bigger lungs may be even more vital to life than here on earth, it’s conceivable that the most successful larger species on other planets will be more horse or cow like than bipedal humanoids. If you have the combination of a thinner atmosphere and less gravity, as is the case on Mars for instance, then you can imagine horse like beings galloping and leaping across the surface of the planet. Perhaps, the Centaur springs to mind half upright and yet with four legs like the horse. Although, the actual genesis for this mythological creature was when bipedal ancient tribal people first saw humans mounted on horses and thought that they were one creature. This was reported as occurring in ancient Greece during the Minoan period. Similarly, it is thought that the Aztecs had the same misapprehension in regard to the Spanish cavalry mounted conquistadors. But, what about our space horses?
Just imagine watching a horse race on Mars in the future, as these noble beasts prance upon the red planet in search of victory. Bets on horses such as these would a whole new experience. Race distances could be much longer, as horse speeds would be much faster. And, just picture the jumps’ race; and the cow jumped over the moon and the dish ran away with the spoon. Colonising space, may, indeed, mean an expansion of the horse racing industry. Jockeys riding in Hong Kong, Mars Station and the Moon! Would jockeys have to worry about weight any more, could saunas become a thing of the past? Stewards would face new challenges, I am sure, jockeys interfering with their opposing jockeys’ space helmets. Horse trainers feeding their ponies new substances found on Mars; but it would be a hell of a lot of fun.