Ah dogs... have a few stories.....
I was out on a ride years ago with my friend on her greenbroke 4 year old. The golden retriever at one of the houses neighbouring the trails took off out of its yard and came after us. The 4 year old was quite freaked out and I charged my horse at it. Most dogs run like hell when 1200lbs of horse comes bearing down on them, but this one stopped, turned around, and tried to dart around my horse to get to the younger one. The owner, meanwhile, was shouting at it but it couldn't have cared less. My horse and I were facing down the dog, slowly push the dog back towards its house, using side-to-side cutting horse maneuvers to prevent the bloody thing from running round behind us, either for the 4-year old or my mare's heels (although if it had tried the latter, it would have been a harsh lesson as my horse knows how to aim a kick), and all the while the owner was screaming at it ineffectively.
Another time, myself and another rider were on trails in Forestry Commission land, with thick walls of trees on either side, when this German shepherd came tearing round the corner, off leash, flat out run. The other rider's horse spooked and the dog went after them, at which point the horse started bucking and kicking out at the dog. The rider came off (she was fine). This all happened within seconds. As she was coming off, I finally reacted and ran my horse at the dog. That changed the dog's mind about chasing the other horse and it veered off, back to its much-embarrassed and very apologetic owner.
One of the liveries at a barn I used to keep my horse at had a collie puppy, wild and hyper as collie puppies are. Now my horse is usually fine around dogs so long as she's not being hassled, so I don't mind when other liveries have their dogs about. On one afternoon, the puppy was running about while I had my horse tied up outside the barn. I was standing a few feet away from the horse, chatting to someone, when the puppy bounded up and jumped on ME. My horse's ears went flat back, the tail swished, and the threatening hind foot was raised. I thought my horse's protective reaction was cute, except it would have been decidedly less cute if she'd thrown a kick. Puppy went back into its owner's car.
I say 'usually' fine with dogs. She takes each dog as they come, which is pretty smart. At a different yard, the owners had a couple German shepherds who lived in a big kennel on the yard most of the day. When the dogs were out of the kennel, which was not that often as far as I could tell, they were fine with horses. But when they were in the kennel, they would jump up against the side of the kennel and bark and bark and bark at any horse which walked past. As the kennel was on the way to the arena from the stable block, this was a fairly regular occurrence. My horse never spooked or worried about the dogs' antics when we walked past, but you'd feel a very slight, almost unnoticeable, annoyed twitch of ears and tail. She wasn't afraid or upset, but she was probably thinking, "stupid &*^&**^^& dogs won't shut up." In the evenings, the owners let the dogs out for a run. The horses were out late one evening as it was summer, and one of the shepherds ran into my horse's field. It could not have cared less about her and was just sniffing for rabbits in the field, but the horse thought it would finally get its come uppance for the all the jumping and barking, and CHARGED the thing, flat out run, ears back, teeth bared. Poor dog had never been so scared and was through the fence within seconds.