The discovery was made by naturalist Sharon Harris, who said she also spotted one frog with only three legs, the Peterborough Evening Telegraph reported.
"I was amazed to see them swimming about," Harris said. "They are fully grown, but they've got extra limbs growing out their legs."
She contacted the Natural England environmental agency, and amphibian expert Jim Foster inspected the frogs. He said similar cases had been reported in the United States.
"It's hard to say why it's happened, but scientists in the U.S. say this type of limb deformity is caused by a parasite, which invades the frogspawn and disrupts normal growth patterns," Foster said.
Environment Agency spokesman Mansai Tang had another theory about the three-legged hoppers.
"One of the reasons for the smaller, or missing limbs, could be as a result of the warm temperatures we had this winter, leading to tadpoles hatching early," she said.