Your best bet:

1> Call United and have them explain the rules for Pet transport (they have dramatically limited any animal cargo in the cabins.)

2. On the website is a story explaining rules for pet transport.

3. Just to warn you, your dog will probably have to travel as luggage/cargo. YOu'll have to meet with your vet to find the best way to travel.

Unlike my cats, I suspect you have a large dog so that's why UA says he/she can't travel up in the cabin.

Good LUck and hope this direct answer to your question helps.