The latest episode of the American Museum of Natural History's outstanding series "Shelf Life" is about the discovery of the olinguito, a ridiculously adorable species of mammal first identified in 2013 – ninety years after its remains were first collected, in 1923.
The story of the olinguito's discovery is common among newly identified species. In a supplemental article, AMNH says:
On average, more than two decades pass between the first collection and archiving of a new species and its formal description.
What accounts for the delay? For one, the sheer volume of the collections. Major expeditions in the early 20th century routinely brought thousands of specimens into the Museum's collections, and researchers are still playing catch-up. Also, the team bringing back a set of specimens may not necessarily have had the expertise to recognize a new find.