Recently I took my minstrel out to charm one of these things to get experience on my Croc Tear Ring. They must've changed the spawn, because this was level 55 while everywhere else listed them as only being 50-51. Broke my charm pretty often.
The horse would not heal me at all, it's a self-heal only. It seemed to heal itself only when it was below 30%, once it was down to like 1% before it did it - it was pretty random.
So in short, it was not a very helpful pet at all.