A member of the public made the horrifying discovery on Monday on the A71 near Newmains, North Lanarkshire, Scotland.
Details from the dog’s microchip revealed she was a Hungarian Vizsla, who was initially registered in Hungary and last seen by a vet in September 2021.
It is not currently known how the dog came to be in Scotland or how she died, due to the level of decomposition.
The Scottish SPCA said they are investigating but cannot comment further due to the ongoing probe.
Find Jeff Scotland, an animal welfare group, said they are appealing for anyone with information to contact the authorities.
They told the Daily Record: “We were contacted today by a dog rescue friend from Lanarkshire Trap and Scan Crew who had the upsetting task of scanning a deceased dog found in a wooden box, nailed shut and dumped in a lay-by.
“This may have been someone who has sadly lost their beloved pet through illness or accident and was not able to afford to make proper provision for the dog.
“However this is not the way to deal with a situation like this. We also don’t want to consider: was she alive when placed in the box?
“Was this a deliberate act of cruelty? If so, those responsible need to be found ASAP.”
READ MORE: Brexit LIVE: Sadiq Khan’s plot EXPOSED as he admits plan to rejoin EU
Find Jeff Scotland added: “This is so distressing and disrespectful for the poor dog to be discarded like rubbish.
“This poor girl was a Hungarian Vizsla, with a Hungarian microchip showing which vet implanted her chip.
“Investigations are ongoing however if someone happens to know of a dog fitting this description who has suddenly not been seen with their owner or perhaps allegedly gone missing etc, it would be good to know, just in case any light can be shed on this sad situation.”