if you're some alien species c. 600 ce and you look down at the biome on this planet, you're just going to see humans and horses as two different animal species that inhabit the planet. you might notice a sort of symbiotic relationship, but you've ultimately got two animals in front of you, and you may be curious as to how they're related.
you wouldn't even know they're both mammals, a priori.
you'd have to cut us both open to figure us out. and, why wouldn't you try to splice us together, to see if it works?