My friend who has a doggie daycare has a very easy solution to belligerent dogs. She puts them out with her mostly gentle but alpha Great Pyrenees bitch, who promptly sits on them until they quit it. 
This is normal Basenji play. I have two, a male and female and they both do the same. In fact, when they engage in the play of chase they also go for the hind legs. This is not aggression but Basenji’s are known to play rough. They are smart enough to know when to be gentle with puppies from what I have experienced.
When I play with them they grab my arm with their mouths and hold it, but let go or gently hold my arm. As puppies it can be a lot more painful for the owners when they play because their teeth are like needles and may result in some scratches. Both my dogs do the same thing at the dog park not only to each other but to their fur buddies when they play.
It’s always good took keep an eye out for other breeds that may not understand the rough play or younger dogs that may get scared from the rough play. My dogs like to play with bigger dogs because they can handle it a lot better. I always communicate to other dogs owners coming in that my dogs play rough and let them make the choice as to what side to go in. Basenji’s will not back down in a fight, so make sure that they are socialized and familiar with the dogs the play with. Dogs are dogs and can have fights. But Basenji’s never forget.
I hope this helps. 
We had this problem with our Basenji too when she was younger. She's more jumpy and in your face playing with other dogs and they don't always like it. She had issues where dogs would get more aggressive with her because of how much she bothered them. We had to be very vigilant in the dog park and leave when it was time. HOWEVER, I am happy to say that she grew out of this. She is significantly less "aggressive" (not ever real agrees just playing rough) now. Around the year and a half mark we noticed a change in her. I do sometimes wonder if it has to do with us staying home more due to quarantine. She listens to us a lot better when we're at the dog park and she knows how to stay away from dogs that might not like her. When a dog allows it she will get rougher, but she backs off when they get angry. I don't think she knew how to do this as a puppy, but as she's older now she's much better at it.
Basenji’s personality may vary. My female is more reserved and my male is more social. She is very loving and affectionate and he is the same but not to the level she is. He is easygoing and laid back and she is cautiously playful.
Experience will vary, my girl played with a pit bull puppy for over 8 months and were the best of buddies but out of the blue the pit bull attacked her. It took an act of God to separate them because neither wanted to back down. Thankfully, they did not hurt each other badly, just some scratches and a few bite marks but nothing life threatening. It was crazy. Sadly she never forgot it and she does not play well with that breed anymore.
My theory is that is was a combination hormonal and adolescent behavior but nonetheless it was heartbreaking for both of us owners to witness.
I believe time heals all wounds but it’s on their time.
Cheers
I just had a situation today at the dog park, where my almost 4yr. old (uncut) male was playing rough and running hard with 2 dogs his size that he's known for along time for about 30 minutes with no issues. Then another puppy(6-8 months) small doodle comes in and my boy breaks contact with the other dogs and just goes at the back of his neck hard and the puppy screamed, and honestly it freaked me out. The owner was mad and his puppy was scared, I asked him to please check his dog for injuries as I told him I wanted to take care of it if his dog was injured. My dog had nipped this particular puppy before but not as aggressive as this time. My dogs prey drive is really strong but, I'm honestly not sure what I can do or if I should do anything about this. Been going to this off leash park since he was six months old and no other instances like this before.
Any ideas or feedback would be appreciated.
@mikero First thought… keep an eye on your dog at all times when at the dog park. Stay close. Focus on him, not the other people. Not to say you can’t converse, but always have an eye on your dog. Be ready to jump in when it’s necessary.
My first Basenji, Jengo, was hot and cold. For the most part he avoided other dogs other than sniffing. Once he did, he left. Once in a while it would erupt into a scrap. Usually it was over quick, but there was one time I had to jump in and separate them. Jengo got bitten. The other owner was upset that I grabbed his dog, but so what. His dog was much more aggressive and I was afraid he’d continue to after Jengo if I picked him up. So, I grabbed his dog’s collar till he came over to control him.
I always have my guard up at dog parks. Things change fast and most owners are not paying attention. Also, by always watching your dog, you’ll get good at spotting their body language. You’ll know before he does that he’s about to snap. If you’re close enough you can call him off or grab him fast. Don’t stand in one spot. Follow your dog. He’s why you’re there. Not the other people. Basenjis don't back down. At least… that was my experience.
@jengosmonkey TY for the reply. I usually do keep an eye on him and unfortunately I stepped in too late. It was my fault for not recognizing his posture. For whatever reason anytime this puppy has come to the park Moko has just been mean to him and I don't understand why. Other pups have come to the park with no other aggressive posture. I don't want to have to have to neuter him, and honestly I've heard that doesn't really help.
@mikero I get it. There were dogs that Jengo just didn't like either. I had to have a eye out for who was coming in, who was there and who was leaving. And, what was going on all the time. It was exhausting! He loved to run and be free. I wanted to feed his soul with that, but I always had to keep in mind that things could go side ways. I didn't stop taking him because he loved it.
It wasn't so relaxing for me though.. And that's a problem. Dogs sense our anxiety and react to it. While watching him, the other dogs, who was coming and going all the time... I had to try and mask my anxiety so as not to add to the problem. Dog parks, for me, are work. I didn't mind it, but I had to have my head right. Maybe not all Basenjis are tough at a dog park, but mine definitely challenged me. Still, I'm glad I took him. He loved it.
@mikero, I agree, neutering at this point likely won't change things much. But since your dog is the aggressor, it's on you to keep everyone safe. If he plays well with most dogs and just the one seems to be the issue, then remove him when that dog arrives or don't let him loose when that dog is there. You can perhaps coordinate with the other owner to make it easier. Hopefully it is just the one dog that sets him off for whatever reason. Perhaps something occurred that you missed. Basenjis do hold a grudge, although the bitches are usually worse in that regard.
It's true that many Basenjis won't back down, and that trait can get them into trouble. They are agile and fast, but when their blood is up they may try it on with the wrong dog and can get hurt. If the dog yours is aggressive with is young, there may come a time when the tables turn.
Neutering will not help one bit and could cause problems for the dog in the long term.@eeeefarm has the right of it.
