I am so sorry that happened!!!
It's hard to ever predict how an animal will act. With the exception of one dog, a black shih-Tzu who could barely hurt a fly, we have raised each of our dogs around horses. HOWEVER, though they've all been with us since puppies, there is one that I wouldn't trust. She is a mixed breed who is now 9yo and smart as a heck... but even though she's grown up around little horses and little goats, I don't think she can be trusted.
The flip side, though, in case this can help anyone else, is you may also not be able to trust some horses around dogs, especially little ones. I know a lot of people allow their dogs in and out with the horses, but we don't. Not even the big dogs unless we are supervising. I can just picture a kick or a stomp... We do have LGD's that "should" do well with horses w/o supervision, but I just picture a kick to a hip or leg... and our other dogs are shih-Tzu's, who only think they are tough.
So sorry for what you and your mare.