Investigators called the makeshift, unlicensed shelter at 39th and Ogden an "animal cruelty scandal."
The SPCA says it discovered the complex of stables on abandoned property within the past few weeks, and served a search warrant on Wednesday.
The stables, investigators said, were sitting right next to a set of legitimate stables that meet city codes.
Four sick horses have been taken from the lot, and were being examined by veterinarians at the SPCA to see if they can survive the months of neglect they've endured.
"The horses are emaciated, they're not cared for at all. They're in really poor condition. The whole lot area is covered in feces and mud, and the horses are living in this," said George Bengal of the SPCA.
Investigators are looking for the owner of those horses.
Neighbors say the owner was an older man who moved onto the abandoned lot with the horses early last summer. He built the shelters, neighbors said, and became popular in the community even though he was squatting.
However, by late summer he vanished, abandoning the horses.
"They would come and get the horses, like over the fourth of July, and they would wash them. They would wash the horses every day, walk the horses every day. They would have pony rides for the children in the neighborhood," said Alfie Myrick of West Philadelphia.
The SPCA says, in Philadelphia, anyone can keep a horse, even in the back of their rowhouse, as long as they keep the property clean, provide medical care, food and adequate shelter.