Garmin forerunner here (301) with the heartrate monitor converted to use with horses. Shows breadcrumb trail, hr, mileage, elevation, distance, speed, has a virtual partner you can train with to give you some competition, you can set alarms to beep at you for certain distance during your training and if your horse's hr gets too low/high, and then when you are done, you can plug it into your computer and it downloads all of your info into a training program and shows you charts and averages and such and even shows a very basic map of where you rode.
You can download your route onto Google earth as well, along with a few photos you might have taken along the way (i.e. if you were on a really pretty trail ride somewhere) and post it all on google earth for friends to see.
Oh, it is water resistant for the most part, has a viewing screen with a nightlight and altho it isn't one of the newest models, it certainly offers more for conditioning your horse than most.
My favorite part is the training center software. Some people keep spreadsheets of their conditioning and this does that for you and you can see your horse's (or horses') progress over weeks or months and get instant results as to what his avg recovery rates/distances, speeds, etc are.
I have not found a current GPS/hr monitor unit that does all of this in the newer models. If someone knows of one, I'd be interested in the website or description - but until mine breaks (and I've tried to kill it numerous times by accident), I'm sold on this unit!