And it is, scientists say: evolution.
A scientific team published the genetic makeup of the Australian animal in the scientific journal Nature on Thursday, confirming that its features - which straddle multiple animal classes - are reflected in its DNA.
The research could help explain how mammals, including humans, evolved from reptiles millions of years ago, they said.
"At first glance, the platypus appears as if it was the result of an evolutionary accident," said Francis S. Collins, director of the U.S. National Human Genome Research Institute, which funded the study.
"But as weird as this animal looks, its genome sequence is priceless for understanding how mammalian biological processes evolved," Collins said in a statement.
More than 100 scientists from the United States, Australia, Japan and other nations took part in mapping the genome, using DNA collected from a female platypus named Glennie.
Jenny Graves, an Australian National University genomics expert who co-wrote the paper, said the gene sequencing shows the platypus has a mix that crosses different classifications of animals.
"What we found was the genome, just like the animal, is an amazing amalgam of reptilian and mammal characteristics with quite a few unique platypus characteristics as well," she told Australian Broadcasting Corp.
The platypus is classed as a mammal because it has fur and feeds its young with milk. But it also has bird and reptile features - it lays eggs, has a duck-like bill and webbed feet, it and lives mostly underwater. Males also have spurs on their heels that inject pain-causing venom to ward off mating rivals.
Scientists believe the platypus and humans shared an evolutionary path until about 165 million years ago when the platypus branched off. Unlike other evolving mammals, the platypus retained characteristics of snakes and lizards, Graves said.
By comparing platypus genes to those of humans and other mammals, scientists hope to fill in the gaps about mammals' evolution and better identify species' specific traits.
Unique to Australia, the platypus has confounded observers for centuries. Aboriginal legend explained the notoriously shy animal as the offspring of a duck and an amorous water rat.
When the British Museum received its first specimen in 1798, zoologist George Shaw was so dubious he tried to cut the pelt with scissors to make sure the bill had not been stitched on by a taxidermist.
Platypuses live along most of Australia's east coast and though their numbers are not accurately known they are not considered to be endangered.