Check out these dog wheelchairs.
Here is a home video of Toby the Dog:
And a tiny little Poodle in a wheelchair
But it is not just dogs that use wheelchairs. Check out this cat in a wheelchair:
Scooter the cat in her cat wheelchair
I think that this is great. However, I have seen alot of homemade wheelchairs and I am not sure that is the best approach. If you are going to put your animal in a wheelchair, try to get one which has been professionally made to fit an animal. There are several companies which make and distribute wheelchairs which have been professionally designed for a pet. A professionally made animal wheelchair will be made by someone who has made many wheelchairs before yours. They will know what works and how to fit your dog or cat. It is the humane thing to do.