I've been thinking it over and I may like to get a heart rate monitor for my horse. Since we aren't yet competitive and don't know if we ever will be, I don't want to drop too much money into a gadget.
I have a handheld polar monitor which I have never had a problem with.
However, I have heard that the ones meant for riding (with the electrode patches) do not do well when the horse is actually in motion. Any monitor can/will have trouble reading if you tend to ride with a loose girth, so not sure if that is the issue with the polars or its a too-loose girth.
I ride with a vmax monitor, which does a very good job unless my girth is swinging in the breeze.