There are no breed or size restrictions for pets onboard American Airlines flights. However, all pets must be in an approved carrier and fit comfortably under the seat in front of you. Pets in the cabin can only be on domestic flights and cannot travel to international destinations. In addition, there are certain states where pets are not allowed at all. These states are Hawaii, Alaska, and California (excluding Inyo, Kern, and Los Angeles counties). Pets are also not allowed in the cabin if the flight is going to or from London Heathrow Airport.
What about other airlines?
Delta Airlines has the same restrictions as American Airlines when it comes to pets in cabins. However, pets are not allowed on flights to/from London Heathrow Airport.