It is not uncommon for elderly, injured, or handicapped animals to lose the use of their back or front legs. Nerve damage, DM (Degenerative Myelopathy), cancer, arthritis, injury, accident and more can result in this condition. Fortunately, this does not need to be the end of a dog's life. A dog cart or dog wheelchair can offer an animal a wholesome, healthy, quality life.
If your dog needs a cart, you can get it here: Link
I've seen a local dog with one of these and he seems to do just fine. The little white dog in the picture looks like my Maltese, Charlie. Too cute!
http://www.eddieswheels.com
They've even fitted an alpaca with a "wheelchair."