I remember watching an animal planet special once about a papillon that worked with horses, and spent his days on horseback with his master. He just kind of hopped up on the horses' bum and rode there. I don't know if that is typical of the breed, but you are much more likely to get a halfway athletic dog that can keep up with you in that breed. There are several pap owners on the board who can give more informed opinions on the breed.
If you're still open to a larger breed, pointers, any of the gun dogs really would be excellent choices. Labs, weims, dals, spaniels, etc. If you stick with a close working breed, and make sure to socialize heavily with the horses there shouldn't be any problems (breed related anyway)
Of the larger dogs, I would think the following would work well:
Wirehaired pointing griffon (we have one, he is AWESOME with non poultry livestock and would be perfect for what you are asking of a dog) In Germany they commonly work with riders on horseback, and they are a close working breed which means that while they are hunting/pointing/tracking they work close to their handler and don't wander a half mile away like some other breeds *coughgermanwirehairedpointercough*
Gordon setter. Maybe look into them and see if they do anything for you.
Maybe a vizsla or weimaraner, though they are both pretty active like the dalmations.
Brittany Spaniel?
For smaller breeds, there are smaller spaniels though most of them need to be brushed a couple times a week, or be clipped. I don't know if that is something you are willing to do. My aunt had her american cocker spaniel with her horses all the time, they got along great and he loved trotting alongside the horses on trail rides.
Keeshond? Will require grooming though. No clipping, but brushing.