I, like many of you, value you a human life over that of a horse.
But the way I see it, friends come and go (at least for me), horses can stick around much longer. The relationship between me and my horse, and me people I care about is remarkably different and I'd give up my horse for them in extreme situations (like if I had to move for a sick relative somewhere I couldn't take my horse) but never just because they asked.
If it was a matter of continuing a friendship or keeping a horse I'd probably go with the horse not because of my intense relationship with animals but because its my hobby, my interest, its what I do. Why give up something that is such a central and enjoyable part of my life for a friend that is, in the end, replaceable?