[Carters Mom]

Kelly:

As far as carry-on, each passenger is allowed only one pet. When my daughter and I flew to New Bern, the second plane was small. The maximum number of pets allowed total on the flight was only ONE. That is why we had to leave Cooper with a pet sitter. Even though there were two of us flying, the 2nd leg of our flight was restricted to only one carryon dog.

I would think that the reason that each passenger is only allowed one pet (if the limit hasn't already been exceeded for pets on the flight) is because they are supposed to be in their carrier under the seat in front of you. I guess they don't want a second pet in your neighbor's carryon location.