To ask people to be able to control their dogs?

[Heathercob]

I regularly ride my cob, Heather down our local beach with friends (and when the riding school was open there were beach rides twice a week), and we regularly have dogs chasing us. Luckily our horses are generally fine with dogs (we have various dogs on the yard), but Heather did kick a dog once (luckily the dog was unharmed) after it sank it's teeth into her fetlock (ankle). It can be quite unnerving when a dog runs up to our horses, barking aggressively!
Am I bring unreasonable to ask people to pay attention to what their dog is doing, and teach their dog to come to call? I know that dogs chasing horses can cause nasty accidents, and I would also be devastated if my horse killed, or injured someone's dog through either a hard kick, or accidentally stepping on a dog.

YANBU at all. There's a riding school near to us and the DC both ride regularly. There are occasions I take the DC to lessons and take our dogs so I can walk them while the DC have lessons and it's unreal the amount of dog owners who'll knowingly walk their dogs alongside the riding school with no control and laugh "oh she's lovely really" as little Sandy the westie bites the horses hooves. It's not funny at all and there should be a huge emphasis put on dog owners to have their dogs trained thoroughly.