Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
I can understand how heartbroken your must have been to see your beloved dog Chiri in such horrible condition. Without being able to examine her and run some tests, and without any history of her breed, what led to her paralysis and whether it was sudden or slowly progressive I cannot be definitive, but I can give you some ideas about what can cause rear leg paralysis in dogs.
If she were allowed to go outdoors unsupervised trauma to her spine (hit by car, a fall) is a possibility but it should leave some external signs such as scrapes, wounds or bleeding, which you don't mention.
Another possibility is diskospondylitis, a bacterial or fungal infection of the intervertebral disks and surrounding vertebrae. It is usually caused by an infection somewhere else that has been spread to the vertebrae by the blood supply. It is a painful condition. Treatment is usually antibiotics or antifungals for at least 8 to 12 weeks, sometimes as long as a year. Prognosis depends upon the amount of damage done and the organism causing the infection. In some cases the paralysis is permanent if the pressure on her spinal cord has been present so long there is nerve death rather than just inflammation and bruising.
Intervertebral Disc Disease happens when the spongy disc or disc material between the vertebrae slips up and compresses the spinal cord causing pain and weakness or paralysis. It can be treated with anti-inflammatories and rest or surgery depending upon how much function is lost and how much pain the patient is in. Prognosis is much better if the patient's sense of deep pain (these dogs can feel their feet when they are pinched, but cannot move them) remains intact and if surgery is done quickly in the cases that require it. It does not usually cause a fever. If she had been paralyzed for a while it is possible to see muscle mass decrease to a large degree due to lack of nerve stimulation, thus this is possible with her given her emaciation.
Fibrocartilaginous Emboli is a piece of a degenerated intervertebral disc that has broken off and lodged in a small artery or vein near the spinal cord blocking the blood supply to her spinal cord and degeneration of the cord. It is often one sided, only painful the first few minutes to hours, and doesn't cause a fever. Signs don't progress after 24 hours unless another emboli is thrown. Prognosis depends upon how much damage is done initially. Recovery is slow and gradual, the most improvement happens between day 21 and 42, and may not happen at all if her sense of pain perception is lost. This seems less likely with her given how severely she was affected.
Another less likely cause of her condition was an Aortic Thromboembolism (saddle thrombus). This is a blood clot that formed in the heart, usually secondary to a primary heart disease called cardiomyopathy, and has lodged in the aorta usually cutting off the circulation to her rear legs. This condition is very painful. It has a higher incidence in males but females can be affected too. Temperature of the affected limbs is sub-normal. The pads on her rear feet should have felt cooler and looked darker in color compared to her front foot pads due to poor circulation. We see this much more commonly in cats than dogs, but it is possible.
Another possibility if she did not seem painful is a condition called ascending myelopathy. This is a progressive degeneration of the spinal nerves that begins with incoordination of the rear legs then progresses to loss of urine and stool control (continence). These pups will lose muscle mass as the nerves degenerate, so this is a good possibility with her given that she lost muscle mass too.
The top breeds that we see myelopathy in are German Shepherd, Corgi, Boxer, Rhodesian Ridgeback, Chesapeake Bay Retriever, but any breed can have some incidences of the disease.
Finally a spinal cord tumor or tumor of the support tissues (bones or fibrous tissue) around the spinal cord could lead to paralysis and loss of muscle mass/function.
If she was a younger to middle aged dog the two most likely primary possible causes of her condition were intervertebral disc disease or myelopathy.
Whatever occurred with your girl given her very poor condition and obvious paralysis you made the most humane choice for her, as difficult and heartbreaking as is must have been. My deepest condolences on your loss and your having to deal with something that you were completely unprepared for, my heart goes out to you.