My dog is very friendly and mellow, he's just a little over 1 1/2 yrs. now. He's usually the one who gets chased at the park and he actually invites it and loves to run, he's fast, everyone comments, and he can keep his distance if he want to with any other dog so far.
But he's very trusting and will slow down or change directions sometimes in total play mode, and he's received a couple of minor nips in the past from a couple of other dogs that weren't that friendly and not regular visitors to the park. Since then I watch and observe, some dogs will look like they're growling or barking and appear to have their mouths open and lunging, but they never bite or attack.
Recently he started being chased a few times by a younger female hunting dog, she was one to show body movement like she wanted to bite his rear in the chase, but never did. I let him deal naturally with the other dogs because he has to learn to deal with them and read their behavior too.
Well, this past time we were watching him being chased by this dog and the owner was right there. That dog was being pretty "assertive" and mine decided it wasn't playful anymore and slowed down wanting to stop the chase. This other dog started working her way up to his neck now with her mouth open, and for the first time ever, we saw our dog bark, growl and bare his teeth at her. It was enough to make her stop her "attack" and back off a moment. He immediately walked over to us and the owner called her and they went on their way.
I was happy to see that he'll stand his ground if he feels threatened, he generally used to always kind of submit to other dogs, stop and tuck his butt, let them sniff and the majority of them back off as normal when one dog submits. There was a female shepherd who was very aggressive with him when he was younger and me and the owner had to step in and get control of the dog verbally because even though he submitted she was over him looking like she wanted to rip his throat out. We avoid that dog now if we see her in the area, I don't need him getting a serious bite or becoming shy around dogs due to fear.
All in all, the majority of the days at the park are very enjoyable and uneventful, most of the dogs that come there regularly are friendly and play nice. We consider each trip to the park as a learning opportunity for him.