I say, and with God's success: The leftover water of the horse is pure and purifying for ritual impurity, as it removes actual impurity from clothing and the body, and it removes legal impurity, which is ritual impurity and major impurity. It is permissible to perform ablution and bathing with it; because its saliva is derived from its flesh, and its flesh is pure, and its prohibition for consumption is due to its honor, as it is a tool for jihad. Thus, its prohibition is akin to the prohibition of human flesh. See: Al-Maraqi, p. 29, and God knows best.