An animal shelter's heartbreaking warning about offering a "free dog to a good home".

It’s a common image on social media and ad sites like Gumtree – “Free dog to good home”.

But now, a Queensland animal rescue shelter is warning people to stop giving away their unwanted pets online.

When Trixie’s owner could no longer care for her, he gave her away for free to what he thought was a “good home”.

The man in his late twenties advertised his six-month-old staffy/rottweiler cross on a Facebook sale page, but after the new owners collected Trixie, they immediately blocked the previous owner on the site.

A fortnight later, Trixie turned up at a Bundaberg pound with a severely swollen face and horrific facial injuries, after being found as a stray dog by the local council’s animal control team.

Posting on Facebook, Brisbane’s Happy Tails Animal Rescue warned pet owners that it’s best to go through a rescue group while re-homing a pet, to ensure the animal actually goes to a ‘good home’.

“Always go through a rescue group or shelter… you never can be sure who you are giving your precious pet to otherwise!” the rescue group who collected Trixie from the pound wrote.

“She [Trixie] has already seen the absolute horrors this world has to offer,” they added.

“We cannot even fathom the horror and pain this girl endured in those 2 weeks after she was given to a ‘good home’.”

From looking at the state of Trixie’s injuries, the rescue believes Trixie was used as a ‘bait dog’ to train fighting dogs.

Trixie's injuries were consistent with dogs used as bait dogs. Image: Facebook.

“We can’t know for sure but pound staff and local rescue group founder said they are fairly confident her injuries were sustained from dog fighting, her face had been mauled repeatedly with the scars at different stages of healing. [It’s] consistent with dogs that are used as bait dogs,” Mykeala Campanini, President of Happy Tails Animal Rescue Inc. told Yahoo7.

The animal rescue drove six hours to collect the puppy and have since provided with her medical care.

Trixie has now been placed in a temporary foster home, where she will recover from her physical injuries in a safe and loving environment before she is permanently re-homed.