See Spot Fly.
Fly, Spot, Fly.
Earn Miles, Spot! in United Pet Class

Now pets are eligible for top-notch travel perks, just like their owners: United Mileage PlusŪ members can earn miles for flying with companions who bark, meow and chirp.

To serve the needs of United customers who are pet owners and committed animal lovers, United introduces United Pet Class. The program offers Mileage Plus members the chance to earn 1,200 bonus miles when their pets fly with them.

http://www.united.com/page/article/0,6722,51201,00.html