[Magus]

I would just ignore them and have the paperwork handy. Not a lot else you can do until if/when they were to try and lodge a complaint with the powers that be. I get that it being a fox instead of a typical pet has you more worried about the authorities examining you more closely but if you have all the legal paperwork to have the fox I don't see how anything can come of it if/when your wacky neighbor complains.

I guess you could preemptively contact whatever government office would be involved and get advice from them ahead of time about what your rights are and what their procedures are if someone lodges a complaint.