It will help reduce blur from you moving whilst holding it, but will do nothing for motion blur in the subject.
IS can open up some extra options to you, but it's less useful for stock standard action shots where you want to freeze the subject.
You won't want motion blur shooting horses and if you're indoors you'll need the 2.8 as it gets very dark in the winter months. You can't use flash..
Edit:: What body are you using?
Hi and thank you for your time
I'm looking at the Canon 70-200 lens... just not sure if I need to get the IS one or not? Being a $1000 difference, I wish money wasn't an issue.. I mainly interested in shooting equine sports, with some be indoor at night.
Unless the arena is very well lit, and they often are not, you will find even an F2.8 lens struggles at night.
As Gary said, a faster lens is king here.
I shoot a lot of indoor equestrian. Pretty well every arena is poorly lit. I use a70-200 2.8 with the VR (IS) turned off.
In the winter it's at 2.8 1/250th ISO 6400 pretty much the starting point. I have shot with a 7D and fast prime but I need the flexibility of a zoom. Some arenas have a little daylight added to fluorescent, may help in keeping the ISO down.
The AF is terrible compared to the Canon and in low light you need the best you can affordlook at the tamron, I think it has better light transmittion, and is cheaper
its the t value
Nikon have a 200mm f/2, not sure if canon have similar.
They do, and it is absolutely AWESOME.

IS is a hindrance for anything other than static low light shots, IMO. When it spins up it introduces blur, and for motion it helps little (even the horizontal panning detection isn't all it's cracked up to be).