"This is fantasy becoming reality," Gilberto Tozzi, director of the Center of Natural Sciences in Prato, told The Associated Press. "The unicorn has always been a mythological animal."
The 1-year-old Roe Deer - nicknamed "Unicorn" - was born in captivity in the research center's park in the Tuscan town of Prato, near Florence, Tozzi said.
He is believed to have been born with a genetic flaw; his twin has two horns.
Calling it the first time he has seen such a case, Tozzi said such anomalies among deer may have inspired the myth of the unicorn.
The unicorn, a horse-like creature with magical healing powers, has appeared in legends and stories throughout history, from ancient and medieval texts to the adventures of Harry Potter.
"This shows that even in past times, there could have been animals with this anomaly," he said by telephone. "It's not like they dreamed it up."
Single-horned deer are rare but not unheard of - but even more unusual is the central positioning of the horn, experts said.
"Generally, the horn is on one side (of the head) rather than being at the center. This looks like a complex case," said Fulvio Fraticelli, scientific director of Rome's zoo. He said the position of the horn could also be the result of a trauma early in the animal's life.
Other mammals are believed to contribute to the myth of the unicorn, including the narwhal, a whale with a long, spiraling tusk.