Just google "equine heart rate monitors" and Polar should pop up but also Garmin. I use a Garmin Forerunner 305 that I (easily) converted for use on my horse. I upload all my data frequently and have had great training and competition records for the last few years, including maps.
There are several threads on this topic already so take a look to see what is out there and what people prefer.
I like the V-Max HRM. They are sold thru a variety of dealers. Here is one of them. Scoll way down this page and you will find the basic Equine HRM for $120. If you have a Garmin Forerunner 301 you can buy a transmitter kit to put on your horse and use the Forerunner as the receiver. Me, all I need is the basic model which comes with a very basic receiver.