...that you know of.

There's multiple ways an animal can be allowed on board. We'd have to probably be specific what animal we are referring to (in-cabin pet, service animal, emotional support animal).

I travelled with a pet cat with UA on EWR-PEK, and the again PEK-EWR a few years later. Both times she was tucked under the seat in front of me.
I am pretty sure most weren't aware of her, including the passengers nearby.