Originally Posted by narvik

...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.