Belgian shepherd would fit with your needs. I have one (groenendael variety): easy to train, brilliant around the horses (no chasing them and out of their hooves), guardian but without agressivity, loyal and athletic enough to follow on rides. And they are gorgeous IMHO!
Some BS have a reputation of being hyper and hard to manage, or need excessive amount of exercise to remain sane. Mine has actually a very laid back personality, so I guess you have to find the right breeder for what you need.