The terrier was heard squealing in Burgoyne Road by a passerby who wrapped her up in their jacket and took her home.
Named Puzzle, she weighed just 2.9 ounces when she was found last month and her eyes were still closed.
An RSPCA spokesperson said:
“Vets checked her over as she had a nasty cut on her head and some scuffs on her nose but thankfully she wasn’t seriously injured. Due to where she was found and her injuries, they believe she’d been thrown from a moving vehicle.
“It’s shocking to think that somebody could take her away from her mother just a few days after she was born and chuck her out of a car onto the road like a bit of rubbish. She’s lucky to be alive.
“We’d like to say a huge thank you to the member of the public who heard her distressed cries and rescued her to take her to the vets; I’m sure they saved her life.”
An RSPCA Animal Rescue Volunteer went to collect Puzzle from the vets and took her to RSPCA Inspector Leanna Hone who is now hand-rearing her.
Leanna said: “Puzzle is doing really well and is getting stronger and stronger every day, which is great.
“She’s feeding every two to three hours so it’s a full-time job taking care of her!
“Thankfully, the wounds on her head and nose are starting to heal so we’re relieved that she hasn’t suffered any serious damage.”
Puzzle has a cleft palate, a split in the top lip and roof of the mouth, which the RSPCA believes is why she was abandoned.
Inspector Kitchin added: “We believe Puzzle may have been abandoned because of her lip. It’s a birth defect and can affect the way a puppy feeds so she may have been struggling to feed from her mum.
“We fear that her owners may have abandoned her instead of seeking veterinary treatment due to the costs associated with any corrective surgery.”
Although the cleft lip is impacting Puzzle’s feeding, Leanna has been successfully hand-feeding her.
Inspector Kitchin added: “I’d like to hear from anyone who recognises this puppy or who may have information about where she’s come from or who was responsible for dumping her.
“As well as being an incredibly irresponsible and cruel thing to do to this poor pup, I’m also incredibly worried about her mum and her other litter-mates, and their welfare.”
Anyone with information should contact the RSPCA appeal line on 0300 123 8018, quoting reference 1245732.
The RSPCA responded to 20,999 reports about animals being abandoned last year, which is more than in 2022, 2021 and 2020.