Cargo vs checked on UA
 

United gives you two options to transport a dog. One is as checked luggage, the other is as cargo. Transporting a dog using the cargo option is a lot more expensive (I was quoted around $600 or more for TATL).
Why would you choose cargo instead of the checked luggage option since the animal will be transported on the same plane anyways.

AFAIK cargo is when you're not on the plane with the animal, i.e. shipping the animal. Checked baggage applies only when you're a passenger on the flight, so cargo would apply otherwise.

You may also wish to review cepheid's pet thread on the UA board.
http://www.flyertalk.com/forum/unite...dden-info.html