DE is not effective when it gets wet. I use it in my garden for roly polys; it must be renewed after watering or a rain. I'm sure there are lots of old remedies for things, and some may work. But parasites in horses may not be where you want to begin experimenting. Make an appointment for your new horse with the vet: floating teeth, fecal count for parasites, his recommendation for a good farrier, and ask about a source of good horse hay (cow hay is not suitable for horses).