I'm a dog lover but the dog (maybe not the bitches) does have a mean streak. Maybe with a responsible owner he might change but sadly its unlikely. I too hope that the horses are okay now.
I hate to say it but if the owner was so irresponsible as to let her dogs run free like that then it's unlikely she'll have insurance.
As it is I hope you reported the incident to the Police as the owner should be prosecuted. Whilst their are bad dogs there are even worse owners.