I found this from Southwest:
http://www.southwest.com/travel_center/animals.html

Pet Policy

For a list of key topics please visit our Frequently Asked Questions
for Pets!

General information on traveling with pets:

Southwest Airlines accepts small vaccinated domestic cats and dogs. All
in-cabin pets must be carried in an appropriate carrier, as indicated
below and are subject to a $75 Pet Fare each way per pet carrier. The
pet carrier counts as the Customer's carryon bag.

Reservations for small cats or dogs can only be booked by calling
Southwest Airlines at 1-800-I-FLY-SWA (1-800-435-9792). Pets will
be accepted on a first-come, first-served basis until capacity is
reached. The Pet Fare must be collected at the ticket counter and is
nonrefundable and may not be applied toward future-date travel if
unused. Customers traveling with a cat or dog must check in pet at
the airport ticket counter and pay the Pet Fare before going to the
departure gate. Customers can still secure their boarding pass online,
at curbside checkin, the airport kiosk, or the ticket counter.

We maintain the right to refuse acceptance of a cat or dog exhibiting
aggressive behavior or any other characteristics that appear
incompatible with air travel.

The following guidelines apply to Customers bringing a cat or dog:

* Southwest Airlines does not accept pets (cats or dogs) traveling
  without a Customer.
* Pets are not permitted to travel with Unaccompanied Minors.

Please note: These requirements do not apply to assistance animals or
emotional support animals accompanying a person with a disability. For
more information about assistance animals or emotional support animals,
please visit www.southwest.com/travel_center/disability.html.

All pet carriers are subject to being searched by the Transportation
Security Administration, other airport security authorities, as well
as searches by Airline personnel.

Note: When passing through security, Customers will be asked to hold
the pet and walk through the screening device while the carrier is
x-rayed.

In-cabin Pet Kennels:

Soft-sided and hard-sided carriers specifically designed as pet
carriers are acceptable. The carriers must be leak-proof and well
ventilated. The pet carrier must be small enough to fit under the seat
in front of the Customer and be stowed in accordance with Federal
Aviation Administration regulations.

* Southwest Airlines allows only one pet carrier per ticketed Customer.
* The carrier may contain two (2) cats or dogs and must be of the same
  species per carrier.
* The cat or dog must be completely inside the pet carrier and be able
  to stand up and move around the carrier with ease.
* Pets must be secured in the pet carrier at all times while in the
  gate area, during boarding/deplaning and they must remain in the
  carrier for the entire duration of the flight. Failure to follow
  this requirement may result in denial of transportation of the pet
  onboard Southwest Airlines.
* The under-seat dimensions are as follows:
  o Window Seat: 19"L x 14"W x 8.25"H
  o Middle Seat: 19"L x 19"W x 8.25"H
  o Aisle Seat: 19"L x 14"W x 8.25"H
However, please keep in mind that some seats might be slightly smaller
in width.

In-cabin Pet Restrictions:

* Customers traveling with a pet may not occupy an Exit Seat or a seat
  with no forward under-seat stowage
* Southwest Airlines will only carry pets in the cabin; pet carriers
  are not permitted as checked baggage.
* Advanced pet arrangements must be made by calling Southwest Airlines
  Customer Support and Services at 1-800-I-FLY-SWA (1-800-435-9792).
* There will be no more than five (5) scheduled pet carriers per
  scheduled flight. However, from time to time, circumstances may allow
  for more (or fewer) than five (5) pet carriers per scheduled flight.
* Cats and dogs must be at least eight (8) weeks old for travel.
* The animals must be harmless, inoffensive, odorless, and require no
  attention during flight.
* Cats and dogs must remain in the carrier (including head and tail)
  and the carrier must be stowed under the seat in front of the
  Customer (Owner) during the entire duration of the flight.
* Southwest Airlines will not be responsible if a Customer misses a
  flight due to the need to take the cat or dog to an outside relief
  area. Flight departures will not be delayed or held in order for
  connecting Customers to take a pet to an animal relief area.
* If the pet becomes ill during the flight, oxygen or other first aid
  procedures will not be administered.
* In the event of an emergency, an oxygen mask may not be available for
  the cat or dog.
* Southwest Airlines assumes no liability for the health or wellbeing
  of carryon pets.
* Southwest Airlines will not accept pet remains in the cabin of the
  aircraft.

[Moderator's note: Some points above might be important to be aware of
when travelling with ferrets, but please keep in mind that Southwest
doesn't carry ferrets at all and other airlines have their own, and
perhaps differing, guidelines. Guidelines also differ if animals are
shipped with baggage instead of in the cabin. Not that airlines allow
ferrets in the cabin these days anyhow :-( BIG]

[Posted in FML 6446]