Camera Lens for taking pictures of horses?

I have a Nikon D3100 what lens would be best for taking photos of horses?

It really depends on what you are wanting to photograph them doing.
for portraits and up close work your kit lens will work nicely. The 50mm f/1.8G would also be an amazing lens for you if you are shooting at a "normal" distance.
If you are wanting zoom and to be able to shoot while they are showing or competing the 70-200 f/2.8 VR would be the ideal. Probably not in the budget though.
Sigma makes one at $1400 that is OS which is the same as Nikon's VR. And one at $975 or so that is not OS/VR. OUTSTANDING LENSES.

Still out of the budget? The Nikon 55-200 or 55-300VR lenses are pretty great entry level lenses.

Check out their site:
http://imaging.nikon.com/...lity02.htm

Also check out some of the review for the lenses (also prices looks pretty good):
http://amzn.to/....to/oe43VU

I've been at this business a long time and used to shoot rodeo and horses. I found there's no best except perhaps for low light. Lots of the indoor arenas, as you probably know, really lack for light.
There you'll want something fast say f/1.8 or so.

However most of what you do will depend on your ability to operate both camera and lens. If you can't do that you're never going to get good results. Skill is a matter of learning. No camera or lens will do everything for you - you have to know what you're doing first then ask the camera and lens to perform what you want it to do.

You really should be asking about what skill set you need as opposed to the lens.