Beau, the dog, was found living in "filthy conditions" with the other dogs in June 2019 - the dogs were found in poor health with no bedding, food or water in a makeshift kennel built in a barn.
The RSPCA said that the dogs were surrounded by and stood in their own faeces.
The dogs were taken into RSPCA care where they were treated for a range of serious bacterial infections and many were put on a special diet to increase their body weight.
Animal rescuer inspector Thomas Hutton, who led the investigation, decided to offer one spaniel a home himself.
Thomas said: "I had two spaniels as pets anyway and Beau seemed such a nice character we decided to take him on - he immediately made a great addition to the house.
"He was in a poor state when we rescued him but he has come on leaps and bounds and as he enjoyed agility classes my partner, Bronia MacMillan, decided to enter him in a competition.
"Last month we took him to his first contest in Staffordshire - as they had not been able to take place due to Covid restrictions - and not only did he come first in one competition but he also came second and third in two others.
"We are so proud of him and how far he has come. Beau is a completely different dog to the one I rescued when he was frightened and shy and in poor health."
A man was given a lifetime ban on keeping animals and was jailed for 16 weeks, suspended for two years, after admitting a range of animal welfare offences after the dogs were discovered and rescued.