You also need to put the good dog/bad dog categories aside sometimes...dogs can and do change habits, and sometimes for no apparent reason.
I was riding about three weeks ago and passed a house that has a blue heeler named Digger. This dog is so low-key, the only times he really even barks at me is if I catch him napping and my clicking gears startle him awake. Most of the time he totally ignores me. Well, this last time as I went by I saw my neighbor out and waved at him and well, things were rather ho -hum. I had completely passed his house and was even down the road a ways when I was startled to hear that old familiar claws-in-the-road-coming-up-fast sound of a dog chasing me. I turned to have a look and there was Digger coming up on my back wheel, and his owner was standing in the road shouting at him to get back there. The dog completely ignored this, but even then I didn't think he was doing more than just running with me, not AT me. I turned back towards the guy thinking the dog would follow and I'd save him some walking. He was scolding the dog and I was just telling him not to worry, in all the years of riding past the place I'd never had any trouble with his dog...when Digger locked onto my ankle. Luckily I had on heavyweight winter tights and he didn't get a good grip because in the next second his owner reached down and cleanly jerked his butt off the ground by his collar (which was equally shocking for me and could have gotten him a good nip, too). I think the feelings of shock and anger were pretty well divided, but I had the suspicion that my pain was nothing compared to what that dog was in for. After reassuring my neighbor that no harm was done, I decided that, upsetting as this all was, I didn't want to hang around and watch a dog beating and rode on.
I still wonder what the deal was with that dog...I've been by there since and it's like nothing happened. When they get older they can do some freaky things unexpectedly, and maybe I'm wrong thinking Digger is a male (not going to ask the dog)--maybe it's a female with pups or something...who knows.