Details of the surprise find in a collection at the University of Aberdeen's zoology museum in 2008 have just been revealed.
The Jerdon's Courser egg is 2-3cm long and is similar to the size of a small duck's egg, but it took five years to establish the egg's identity, including DNA analysis.
Breeding habits of the south Indian bird called Jerdon's Courser, or Rhinoptilus bitorquatus, are unknown and an egg has never been found by an ornithologist.
Experts hope that the discovery will help conservationists identify the species' egg in their work to save it from extinction.
Dr Alan Knox, the university's emeritus head of museums, said: "I was looking through drawers of uncatalogued eggs in the university's zoology museum when I spotted an egg labelled as belonging to this species.
"It was one of those eureka moments - finding something nobody else knows about, something so rare and exciting. I could hardly believe my eyes.
"My first question was how do you identify something that the books say has never been discovered?
"Very little is actually known about this extremely rare bird, which inhabits an area only a few kilometres wide.
"It was seen only a few times in southern India before it was presumed extinct around 1900. It was rediscovered in 1986 but is listed as a critically-endangered species.
"Without any others to compare it with, the process of correctly identifying the egg was a challenge."
The egg was taken to the Natural History Museum in Tring, Hertfordshire, home of the world's largest egg collection where similarities with other courser species eggs were found.
DNA was extracted from the dried-up membrane, scraped from the inside of the egg, and a match was found with DNA from the toe of a 140-year-old Jerdon's Courser at the museum in Tring.
The small collection where the egg was discovered was put together, probably in 1917, by a vet called Ernest Meaton who worked at the Kolar Gold Fields east of Bangalore in India, the University said.
Aberdeen Grammar School was gifted the collection in 1919 by a former pupil, George Rose, who bought the collection, and it was given to the university in the 1970s, where it was unexamined until 2008.
Details of the discovery have been published in the Journal of the Bombay Natural History Society.
Once refurbishments at the university's zoology museum are complete, the egg will go on public display.