I spent much of 2002 - 2006 shooting equine events... for my company, and a bunch of magazines & websites... I don't have most of the shots up, but there were a few that I posted from a couple of the events (Wellington's Winter Equestrian Challenge, Rolex's Three-day Eventing, and HITS):
http://www.flickr.com/photos/terry_mercer/3085353883/Shutter Speed 320
Aperture f/4.5
Focal Length 46.8 mm
ISO Speed 200
Exposure Bias 0 EV
Most were shot with a fixed lens (large pocket) camera back then... a Fuji FinePix 4900 or 7000Pro... but I would say that shutter speed is dependent on the lighting and other settings.
http://www.flickr.com/photos/terry_mercer/sets/72157606622201174/with/3085353883/It's all about positioning... distance from the action... the lighting, and timing.
With equine events, the position of the horse, its feet, the riders body position (and their feet) will be closely evaluated. Knowing what they expect & want is key.
Perfect positioning:
http://www.flickr.com/photos/terry_mercer/2746157132/in/set-72157606622201174Not so perfect:
http://www.flickr.com/photos/terry_mercer/2746170506/in/set-72157606622201174 (notice the horses front legs aren't 'even' and don't match?!?!
Unless it's a big name rider, if you are wanting to sell for stock, or generic advertising use... try to catch angles that don't specifically 'identify' the rider:
http://www.flickr.com/photos/terry_mercer/2753152297/in/set-72157606622201174Anyhow, depending on the type & style of events you are shooting... it's AWESOME! If you are shooting 3-day eventing, by yourself a golf cart, segway, or really quiet scooter... because the average course is about 10 acres... carry extra batteries & memory cards with you, and leave the rest of the stuff locked up in your rig (or get a shooting vest to carry some basics with you). I loved shooting those type of events. If you have a 'team mate' (business partner or employee) that shuttles the memory cards, does the post processing while you continue shooting, and can display the finished shots, and take orders right there... there is a pile of money to be made shooting equine events. A mobile processing/printing studio with laptopS and printers, and a few large screen displays of processed photos really drives up sales.
A normal day in the hunter/jumper world was worth about $500 to $5000 in direct sales, and if you had good enough stuff, and the contacts with the magazines... you could more than double those numbers after the fact. There is MORE competition now days, and for most events press passes are each to get... and the really cool thing, is in most places setting up a mobile 'shop' is either free or just the parking fees.
Best of luck...