Before you even get started taking photos of horses, you'll want to know how to be safe around them. If you are not experienced in this area, have someone help you -- better safe than sorry. 
 

1. Find a location to take your photos and clean up any "junk" that won't look good in your photo. While you are cleaning the area, clean your horse and tack too. Clean and polish anything that will be in the photos. If you are in a winter climate zone, the spring with all the muddy areas and woolly mammoth looking horses is not going to be a great photo opportunity. Wait until the horses shed out and trees are leaved out.
2. You'll get the most flattering light for your images if you shoot at the "Golden Hour"  the first and last hour of sunlight in the day -- you know, dawn and dusk. These times produce great lighting for photographs and makes it easy on you too. Want to know when the times are in your area, check here: The Golden Hour Calculator / Sunrise and Sunset information for photographers
3. Get the ears up (pointing forward) so the horse looks good. There are lots of ways to get their ears pointing forward, and one thing I usually bring along are peppermints -- most horses seem to really like them and if you crinkle the wrappers it usually gets their ears up. Added perk, if you have bad breath, the mints come in handy.
4. Have horse-savvy helpers to let you know if something needs adjusting and to keep the horses looking their best. If you haven't got a lot of horses experience, you may think the horse looks great and it really isn't all that flattering. Even with all my years around horses, I still get some images where the horses aren't looking great, so don't worry, you'll get the hang of it.
5. Study your subject and the disciplines you are shooting -- knowledge is power and it will help you get better images. Dressage horse photos should look different than stock horse photos and those are different than Saddlebred photos and those are different than an Amish buggy horse trotting down a road or a draft horse pulling a heavy load. 

Which of these images of the Thoroughbred stallion Sam Lords Castle do you like best?

I'm here to help you develop an eye for horses. I'll be offering some FREE help on getting the right shot at upcoming horse events, let me know if you want to participate.