There's no like hard and fast rules, and I'm def not an expert - it usually takes me quite a few tries of blocking out shapes to get movement to look right.

What I would say though is generally avoid straight lines, keep things curvy! Also look at the stride of the horse, in this case its sort of landing, so coming down in stride, so air would be pushing up against the hair. There's tons of different ways to take that but I'd say try and flick the end third (or more) of the hairs upward to give the impression that they are still falling if that makes sense.

Manes I find quite difficult to shape but personally for this I'd go with it all being elevated off the horses neck sticking upwards (shown in red. If so I'd to shade the part closer to the neck darker - gives the impression its not sticking straight up but the ends are facing towards you a bit. But you could also have it coming down the horses neck but flicking up at the ends (green) or if you wanna be extra fancy have it blown almost against the neck (blue).

Tails are a little easier for me, I usually think about how the dock (bone of the tail) usually is elevated when running and so sticks out from the body and is a bit more rigid, while the rest of the hair will flow with the motion. You could go in any direction you wanted but I quite like having an almost double C shape (see the pic) cause it shows movement nicely.