Katrine Henriksen, 20, a student in the nearby town of Tønsberg, was out riding with two friends when she was noticed an appalling smell.
"I think it's terrible," she told The Local of her grisly experience. "The smell was very strong, so we just followed the smell and then we found them. We took pictures and then went back to the stable owner, who called the police."
She said that the three riders' horses had been terrified by what they had found.
"I think it's because of the smell," she said. "It's very intense, and the horses know when they smell something like death."
Police on Monday combed the scene for clues that could lead to the perpetrator of the crime.
Henriksen said that she believed that the as yet unidentified owners of the animals were to blame.
"I think the owners put them there. Because it's very expensive to deliver them to a veterinarian," she said.
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Here is another picture Henriksen took of her macabre discovery: