Hardened vets and dog wardens were close to tears when they treated a dog with the biggest tumour they had ever seen, a spokesman said.
The dog was discovered in a woman's garden in St Helens, Merseyside, on Thursday.
The animal was taken to a vet, who put it down.
The body has been given to the RSPCA who have appealed for information to help launch a prosecution against the owner.
A council spokesman said: "Dog wardens and vets were close to tears after one of the most horrendous case of neglect they have ever seen. St Helens Council dog wardens were called out yesterday by a concerned member of the public who had taken a stray into her garden.
"It was suffering from one of the biggest tumours ever seen and was unable to sit or lie down due to the size of the tumour.
"The wardens took the dog to a local vet who put the dog to sleep to end its suffering. The vet said that due to the size of the tumour and condition of the dog it had taken many months to grow.
"From the look of the dog's claws it had not walked on a hard surface for at least two years.
"The RSPCA are looking at a prosecution if the owner can be traced.
"They have taken custody of the dog's body, which had no tag or micro-chip."globe