He was rushed for veterinary treatment but could not be saved.
RSPCA inspector Keira Benham said: "The little pup was collapsed and unresponsive. He was soaking wet, struggling to breathe and couldn’t stand.
“The poor little thing was very weak and was covered in fly eggs. He was barely alive when I got to him but I took him straight to the vets.
“Sadly, he was having fits and vets said it was down to a neurological condition so they decided that the kindest thing to do was end his suffering and put him to sleep.”
The black and tan pup had no microchip, collar or ID.
The RSPCA believe he was a ‘teacup’ Yorkie - a miniature Yorkshire terrier - but was dumped due to his neurological problems and health complications.
“This innocent and vulnerable dog had health complications which would require considerable - and costly - veterinary treatment. It’s a sad reality that we see animals dumped like this simply because their owners don’t want to pay,” inspector Benham said.
“The poor little thing needed urgent, specialist treatment but, instead of ensuring he saw a vet and was given every chance to overcome his health problems, he was dumped, like a piece of rubbish and left to die alone and frightened.
“We have launched an investigation and I’d be keen to hear from anyone who may know where this little pup came from or who saw anything suspicious in the area."
The RSPCA said it has 'serious concerns' about the way many dogs, like so-called ‘teacup’ breeds, are selectively bred to look a certain way.
The charity’s dog welfare expert, Lisa Richards, said: “Selectively breeding dogs to have exaggerated physical features - such as extremely small body size - can cause serious health and welfare problems, and may have been the cause of this poor dog’s neurological problems.
“Teacup dogs can also suffer from problems as a direct result of being so small. For example, their ﬁne bones are very vulnerable to stress fractures and they are at risk of painful fractures from minimal trauma such as jumping; they can suffer from dislocation of the kneecap, leading to pain and lameness; and can also suffer from respiratory problems that can severely limit their ability to run and exercise.
“We are concerned that many pedigree and purebred dogs are suffering because they’re bred primarily for how they look rather than with health, welfare and temperament in mind.”
Call 0300 123 8018.