I don’t know anyone with a child named Max. Years ago, I knew one family with a “Maximilian,” named for St. Maximilian Kolbe.
I know two or three families with a “Maribel” or “Isabel” who is called “Bella” or “Belle.”
Don’t know anyone with these pet names, either. We have to remember that this is one Boston vet’s experience, not a statistically-valid nationwide survey.
A good friend of mine named his son Maximillian(Max) and AnneBelle(Belle). I personally don’t care for either of those names. I just laughed seeing those are animal names.