Dyfed-Powys Police has detained a dog in kennels for more than two years as part of an inquiry.
The force is yet to make a decision on whether or not the animal is destroyed, returned to its owner or rehomed.
It would not give full details as to why the animal was being held, saying it related to "an ongoing case".
It has been highlighted by BBC Radio 4's You and Yours programme in an investigation into the costs of enforcing the 1991 Dangerous Dogs Act.
The programme found animals suspected of being pit bull terrier types were routinely detained for periods of longer than 14 weeks.
A senior officer has said forces in England and Wales were spending £3m annually on kennel cost with the length of time courts take to resolve disputes over breeds blamed for the longest delays.
In 2017, just over 2,000 dogs were detained by police, the vast majority under the Dangerous Dogs Act, according to 29 forces in England and Wales which responded to a Freedom of Information request.
Of those, at least 334 were taken because they were suspected of being a dangerous breed, although they did not need to be out of control or a threat.
'Desperate' to cut costs
No further details have been revealed about the Dyfed-Powys Police case but the programme found it was responsible for the longest detention period.
Figures showed the force has paid out £105,246 in fees for kennels, transportation and veterinary costs for 53 animals since 2014.
North Wales Police spent £154,307 on fees in the same period but it has not revealed the number of dogs detained.
Breeds banned under the 1991 act include pit bull terriers. If dogs are found to be a banned breed, they can then be destroyed.
Gareth Pritchard, the National Police Chiefs Council's lead for dangerous dogs, told BBC Radio Wales's Good Morning Wales there had been more than 30 dog related deaths in England and Wales since 2005.
Mr Pritchard, the Deputy Chief Constable for North Wales Police, said there was a "public safety issue" with problem dogs but forces wanted to "speed up the process" of dealing with cases.
"We in policing spend over £3m a year on kennelling costs and we are desperate to reduce those costs and maintain the appropriate animal welfare standards for the dogs that are in our kennels," he said.
South Wales Police said it does not hold any data and Gwent Police did not respond to the request for information.