I sprayed a dog a long time ago while out on a ride when I was a teenager.
I was riding through the countryside in Central Maine when all of a sudden out comes Fido from a house that I passed. I wasn't going very fast, so I tried to outrun this fairly small-sized dog by pedaling a little bit faster. Unfortunately, this didn't work. So I pulled out my quart-sized water bottle and sprayed him in the face with a good-sized squirt. Unfazed, the dog kept going. My second try was no different. My third attempt drained the bottle but brought more water into the dog's face with greater force. With a whimper, the dog gave up and turned back toward its home. I was thirsty during the rest of the ride but no other dogs decided to follow me that day.
Other dogs have chased me since that occaision. One I managed to outrun on a three-mile downhill chase.
At other times, I've scared them away with a loud blast of a whistle or air horn.