It can be, my other half's psychiatrist put it in a letter for our new apartment. I think it's usually something about Emotional Support Animals. They aren't traditional support animals but they help with certain issues.

Course, we would never bring our cats anywhere they would inconvenience other people. The larger one does travel fairly well though, he mostly just flops down and falls asleep. The other one though... he yowls and I would be furious if I was stuck on a plane with him. (I am driving them both cross country next week)

All that said - Im annoyed at people that falsify these sort of things. There's all manner of services online that will print fake documentation for a nominal fee. It's insulting for people with actual issues.

Emotional Support Animals aren't a regulated thing in most states, and even the few where it is semi-regulated so far it's mostly more "homeopathic medicine / food product" ("generally regarded to be safe") than "FDA regulated" (scientifically tested, strictly licensed).