This happened to me when I was probably 12 or 13.

Riding my bike on a fairly familiar route, next to a highway with lots of retail around, so I'd ride through the parking lots. Came near a corner and this dog just comes out of nowhere, growling and snarling like a mad hound. Looked like some sort of sheppard / husky mix. No getting away - he came at me from a front angle. SOMEHOW I managed to dismount and the get bike between me and the monster. We circled around each other like that for what felt like a really long time, all while I'm trying to find some angle where I can get the jump on him and take off, but he was having none of it.

FINALLY a guy in a van comes off the road and eases in between me and the dog. I was grateful as hell for that kind stranger, but I didn't wait around. He gave me the opening and I took off. If the dog gave chase I never noticed. Did NOT look back...