This happened to me about a year or two ago, I had a thread on this forum or FOH about it but I can't find it now. We had a dude who was just discharged from the Navy living below us with his wife. Our dog was just a puppy at the time and even though she was tiny she would run hard through our apartment. We always grabbed her, but as we all know puppys are hard to control at first. Shortly after getting the puppy, this guy came up and was banging on our door. I answered the door to meet said neighbor looking really red and extremely sweaty. He told me it was getting ridiculous and to knock it off. I told him I would and he apologized for being so mad, told me he came up here with the intention of knocking me out.. I should've called the cops, but I thought everything was cool at that point.
Over the next two months, the guy was on my doorstep nearly everyday. I actually started not answering my door altogether, I knew it was him and he was being ridiculous. He would often stop me outside and complain some more. His wife also complained to us in his place, because he "got too mad." They asked if we could play with our dog more during the day so it was tired at night. We complied and our dog was sleeping most nights because it was so worn out. Then we started getting complaints that the dog was too loud during the day, we could not win.
A little bit of a backstory here, at the time my dog was around 9-12 pounds and he told us our dog walked very loudly on our carpeted floors. We asked his wife what was up and she told us he had PTSD and couldn't sleep at all, they had to sleep in separate rooms. She told us he got angry very easily and at this point we were a little afraid.
We went to management who told us we were in the right and as long as we weren't making excessive noise after 9PM we were in the clear - footsteps didn't count as long as it wasn't excessive. He went to management to complain to us several times in which they told him to grow up, that's what he gets for living in an apartment. He called the police on us a few times as well but actually got in trouble because you can't file a complaint for a dog walking around at night.
Eventually they moved out, but it was extremely tense for a long time; the apartment was actually vacant because they moved out but continued to pay their rent, we saw them come every few weeks to pick up a random piece of furniture. Having someone with PTSD living below you and that person has anger issues really scares you. Hope it works out dude.