The long-beaked echidna, which is about twice as big as a common Australian echidna, was thought to have become extinct in Australia in the last ice age - although it still lives in Papua New Guinea.
Dr Kristofer Helgen from the Smithsonian Institution has told Radio Australia he's discovered a stuffed specimen that was only about 100 years old in London's Natural History Museum.
"I recognised...the tag that was used by a particular collector who worked for the Western Australian Museum more than 100 years ago...a famous Australian naturalist named John Tunney," he said.
"He had done a series of pioneering expeditions across northern Australia, from 1901 through 1903.
"But when I saw this tag attached to the long-beaked echidna, it didn't make sense [because] I had no idea that John Tunney had ever gone to New Guinea."
Until now, the only records of the Australian animal were from fossils more than 10,000 years ago, and also from ancient Aboriginal rock art.
But Dr Helgen says the sample at London's Natural History Museum is labelled as coming from Australia.
"When I looked closer, what I could see is the descriptive writing 'Mount Anderson, West Kimberley'," he said.
"I realised, my goodness, is this actually saying that it's actually a long beaked echidna that was collected in Australia? That was phenomenally exciting if that was true."
Researcher have recently rediscovered several Australian animals, including the scaly-tailed possum, last seen in 1917.
Dr Helgen says finding the sample in the museum does raise the prospect that the long-beaked echidnas may still be roaming in remote regions of the Kimberley.
"Ultimately I think that's probably what's the most exciting possibility...that clue that they may still be out there," he said.
"Working with animals all over the world, one thing that I've learned is that it's very easy to overestimate what we know."