Is this normal?

The only way to stop him is for me to pet the dog and tell Loki that he's nice.

what happens if you ignore the behavior?
Molly's behaviorist told me any reaction from me to her reactions to various things simply fuels what she is doing. Even my acknowledging her when I leave the house feeds into her separation issues...I still say bye.
My friend has a boxer that hates white dogs. Any dog she brings into the house is okay (she fosters for the shelter...she's the volunteer director) but she can't bring in any white dogs. Her boxer goes nuts if a white dog even walks past their house. But her boxer is a rescue whom she adopted when the dogs was about 6 so who knows what caused it.

If I ignore it, he'll keep doing it. The owners usually try and walk off to a different part of the park and he will follow them. I don't want people to feel like they have to leave because my dog is being annoying. So I follow them around til I can catch up and pet the dog. Quiet and Leave It don't do anything when he gets in that frame of mind.
 
To my knowledge (we've had him since he was 8 weeks), he's never had any weird experience that would cause him to be afraid of big black dogs.

When Lana was a pup she ONLY would play with white dogs...preferrably white and fully. I guess you can say she only wanted to play with dogs that looked like her.  She didn't bark or anything she just refused to play with them.  She eventually grew out of it.

Emma has a definite preference for terriers, particularly short haired terriers.

Of course, she tends to get along with big dogs better, since she's so rough and tumble. She and her buddy, Jake, the white lab mix, can play for HOURS.

I dunno what I'd do if one of them decided that they didn't like a certain kind of dog.

I work at a doggie daycare and I know exactly what you're talking about. We have at least a couple of the "mediating dogs" pretty much every day. We don't allow them to constantly bark at 2 playing dogs like that because it has the potential to cause a fight. We say their name firmly (that usually stops it because they know they shouldn't be doing that) but if that doesn't work we take them by the collar and lead them somewhere else to distract them.
 
As for hating black dogs, I've seen that too. But only a couple times. One dog comes to mind who goes to the daycare who always plays rough but he absouletely hates big, black dogs and will constantly harass one in the yard. Usually they have to be seperated. I don't know what causes it but my guess would be that it's something they're not used to. Dogs have a tendency to get along better with dogs that look like them. At the daycare dogs that look alike often become fast friends. Of course they play with other breeds too, but dogs of same and similar breeds tend to do better with each other. Not sure why though.

Hmmm, well at least I know he's not completely abnormal then. I just wish I could stop his weird behavior. 99% of the time he's great at dog park, etc--but I don't want him to be annoying.

ORIGINAL: lokis_mom

His other weird quirk is that he gets really upset whenever two dogs are rough-housing too much or especially when one dog is humping another. He runs up barking and afterwards seems to have an extreme dislike for the dogs involved. It's like he's playing a mediator and wants everyone to get along.

 
That is so cute!  Diasy doesn't like it when Duke barks at anything.  She will go crazy on him when he barks, she even bites him!