>>96776845
>outside of the logistics of finding a horse strong enough to carry a 1500 lb space marine
Space GMO horses? Obviously "literally a normal pony" doesent work and would be silly, both aesthetically and logically, but look even at your example image, those are weird genetically modified beasts vaguely shaped like horses.
>sm already run faster than horses
But they dont. Not faster than (insert weird space horse here) anyways.

SM arent faster than horses, theyre not even faster than space-dogs (m8). The universe is full of all manner of weird alien life, lots of it is very physically capable.