it's a person
So if we go with the interpretation of Woyzeck being the horse, this line asks a question, what is the cause of Woyzeck's metamorphosis? If an animal is just a human without the ability or expression of the ability of reason, and Woyzeck finds himself in a that kind of state by the "end" of the play, what was the catalyst for this transformation?
It was being the horse. Woyzeck, for all his lack of propriety and crassness and paranoia is still a person who is worthy of care and respect. But characters like the Captain and the Doctor, and the conditions of living in poverty, having to give up your dignity and happiness for the sake of survival striped Woyzeck of his personhood, which in turn cost him his family. He was not a human, he was a utility. And that is what drove him mad.