Maybe a veterinarian's customer database? They would have to distinguish pet names from humans, but keep a record of all.
Yeah, that's one plausible explanation. (I don't remember the nature of the letter.)
Relatedly, one time I picked up a prescription for a cat. The cat's name was listed as CatFirstName MyLastName. They had another (human) client with that same first and name. It turned out that on my previous visit they had "corrected" that client's record to indicate that he was a cat.
I never figured out what kind of mistake could have led to that.