“has experienced more human interaction than any other human alive” It’s pretty clear the series has shown you in the process of this timeskip what Fushi was doing the whole time, I mean, as if him avoiding all the guardian descendants wasn’t enough proof. He steered away from interacting with any other humans because not only could he not bear to witness more friends dying, but he could never make the kind of bonds again he did with March, Gugu, Tonari, or any of his other vessels. Because of this, his idea of creating relationships and interacting with humans was never really up to speed, not just because he was out of practice but because he chose not to. As for your regeneration complaint, I don’t know why you’re arguing it cause your reasoning doesn’t hold much water. He can easily be poisoned with the body he uses, and especially in the case of knockout, he can’t regenerate himself because he’s unconscious, it’s not hard to understand. The reason Fushi doesn’t have as much care for other animals is because he never made much of a bond with any other animal aside from his new horse, but you can’t really add that into the equation because of the horse’s unique abilities. He’s constantly learning about human emotion, which is why he can show feelings towards humans, whereas animals never exactly taught him much simply. All he knows of them is they share the world with the dominant human species.