Are there any dogs know for being good with horses?

I know there are dogs that are known for being good with cows and dogs that are being good with sheep, but I've never heard of dogs that are good around horses. So yeah... that's my question. Are some dogs know for being calmer or more level headed around horses? If I don't get enough answers here I might re-post in the dog section. Thanks guys :) Oh, and if you can, will you tell me how you introduced your dog(s) to your horse(s)? Did you leave your horse untied and keep your dog on leash? I've never really read anything about that. Thanks guys :)