Two piebald horses were found dumped in a parking space by the Charlton ground entrance
One emaciated body was found at the entrance of the ground in south-east London in late January.
Then just two weeks later on Saturday morning, two more dead piebald horses were found dumped in a nearby parking space.
RSPCA said they had been left overnight and were malnourished, neglected and unkempt, with their ribs showing.
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We see far too many incidents involving dead horses dumped in this way
Inspector Nicholas Wheelhouse
But it was unclear what had caused their death.
Inspector Nicholas Wheelhouse said: "We are very concerned that these horses have been discovered dead so near each other and in such similar circumstances.
"We urge anyone with any information about what happened or who may have owned them to call us, in complete confidence.
First days of the Appleby horse fair Gallery Fri, June 3, 2016
First days of the 2016 Appleby horse fair. The biggest annual gathering of gypsies and travellers held in Appleby Cumbria.
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Police are investigating the incidents and have called for information from the public
"It is very upsetting for passers-by and everyone involved when animal bodies are dumped so callously like this as if they were rubbish, but for us what is even more concerning is finding out what caused their deaths.
"Sadly situations such as this one is not uncommon and we see far too many incidents involving dead horses dumped in this way."
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