But after a rigorous fitness and diet regime she has lost half her body weight in six months and is now fit enough to be rehomed.
Delighted carer Emma Rex said: "She's a completely different dog." Cassie's owner was frail and lonely and the pooch was her only companion, so they would eat meals together.
The lady would cook twice the amount she needed and give half to Cassie or would order double if she was having a takeaway, one for her and one for Cassie.
The pooch's weight rocketed over the years to THREE TIMES what it should have been, sparking fears she was the fattest dog in the world.
When staff at the charity's branch in Kenilworth, Warwickshire, began caring for her they immediately put her on a diet of low calorie dog food and gave her a gentle exercise routine. They got her doing longer walks and then put her on a doggy treadmill and also encouraged her to go swimming and the weight began falling off.
Staff hoped she would lose half her body weight in a year but she has reached that milestone in just six months. She should weigh around three stone but at four-and-half stone she is healthy enough to be rehomed with a single mum and her seven-year-old daughter in Coventry, West Midlands.
Emma said: "It was awful to see the state of her when she came in, none of us had ever seen anything like it. She couldn't move. "We started her off on a diet of small quantities of dry dog food. For a few days she went on hunger strike because she had never eaten dog food before, but eventually she relented and she's made fantastic progress ever since.
"We gradually started encouraging her to walk very short distances, and because she co-operated fully the weight began to drop off at a consistent rate of a couple of pounds a week. "Soon she was able to take very short walks three times a day, and we gradually increased the length of these walks. Then we started hydrotherapy which involved her swimming around which she loved, and also walking her on a treadmill.
"The wonderful thing is she has an absolutely brilliant personality, but before she couldn't show it because she couldn't move. Now she can show her own personality to the full. "She's lovely, so friendly to other people and other dogs and she is brilliant with kids which bodes well for her future. We have all become very attached to her so will be sad to see her go but we're so happy for her."
Emma added that Cassie's new owner has been fully briefed on her history and will continue to feed her just low calorie dry dog food and take her for walks.
Every fortnight she will return to the centre to be weighed as a precaution. Dogs Trust Kenilworth manager Sandra Wilson said: "Unfortunately, when owners treat their pets as if they were humans and feed them the wrong food, they're simply killing them with kindness. "She was at risk of heart disease, diabetes, and arthritis, but it's great to see the progress she has made in a relatively short time and a testament to the hard work of the staff here."