Lindy, a blogger from Wiltshire, revealed her bizarre experience after picking up what she thought was a pedigree pooch.
A certificated purebred St Bernard can fetch as much as £2,000 online.
Lindy was expecting a sweet-natured puppy when she parted with her cash, but was baffled by the adult dog it grew into.
A picture she posted to TikTok and Instagram shows a slobbering hell-raiser licking her window, its snout pressed to the glass and its teeth and gums bared.
Lindy still loves her pooch, whom she described as a “little hooligan”.
It comes after a woman who forked out for a purebred Australian Shepherd was stunned after he turned out very differently, shocking vets.
The American woman, who describes herself as an “authentic, relatable dog mom”, revealed her bizarre experience after picking up the mutt on Craiglist.
A video she posted to TikTok includes heartbreaking footage of him twitching on his side during a trip to the vet.
But she ended her TikTok with snaps of him as a healthy, happy adult dog smiling for the camera.
Whether or not the mutt is an Australian Shepherd, he remains a very good boy.
Another woman who bought an expensive purebred mini “poodle” explained why you should never buy a dog on Facebook.
American influencer Didi Perez revealed her shocking experience after picking up what she thought was a posh pooch on Facebook Marketplace.
The influencer shared adorable snaps of her puppy before it grew into an adult dog.
The nine-second TikTok video includes footage of the little mutt cutely pawing its way across the wooden floorboards of Didi’s living room.
But Didi was left stunned by what became of it as it got bigger.
Footage of the grown-up pooch revealed how it had become a wiry, hairless hound in no way resembling a purebred poodle.
Another dog lover who bought a purebred dachshund was baffled after realising she was scammed in the sale.
A purebred pug bought on Gumtree also turned out to be a giant mutant.
You should only ever buy a dog from a certificated breeder and generally steer clear of purchasing animals online.