The poodle cross was found dumped by the road at Eye, near Peterborough, on Wednesday.
It took two hours for four people to shave off 1.2kg of his matted fur, while he was sedated and on a drip.
When they finished, they found that one of his paws had rotted off.
An RSPCA spokesman said: “It is likely the foot could have been missing for months, if not longer.
“It is thought this was the result of the matting which caused the circulation to stop.
“Without a blood supply his foot effectively died.”
Benji was severely dehydrated and could hardly move when he was found by a group of teenagers who stayed with him until the RSPCA arrived.
RSPCA inspector Carrie O’Riordan thanked the group for looking after Benji after they found him.
“They stayed with him until I arrived and they wrapped him in their shirts to keep him warm and comfortable.
“They even played him music to keep him calm.
“I fear he may not have been found had it not been for them and he could have died a slow and painful death.”
She added: “I was truly horrified when I saw Benji, he didn’t even resemble a dog.
“Apart from the matting, he smelt extremely bad and his fur was coated in faeces, urine and fleas.
“In all my years as an inspector I have never seen a matted dog as bad as this.
“The person who let him get to a condition like this is truly the lowest of the low, there is just no excuse for it.”
Julie Eastham, practice manager at Cromwell Veterinary Group, said: “The vet involved with Benji’s care said that this was the worst case of neglect that she has ever seen.
“It is heartbreaking for all our staff to see such a sad case, poor Benji had been suffering for some time.”
Benji, who is thought to be around seven years old, is now at an RSPCA centre near March, Cambridgeshire, where staff say that he had been “extremely nervous and scared” on his arrival.
He was now “starting to come out of himself and is very friendly and sweet”, they added.
Anyone who recognises Benji or knows his owner should call the RSPCA on 0300 123 8018.