This article is from the Puzzles FAQ, by Chris Cole email@example.com and Matthew Daly firstname.lastname@example.org with numerous contributions by others.
What words have their plurals formed by dropping the final letter?
There are a number of words (e.g., "jinni") where there is a variant
spelling sans the terminal letter (e.g., "jinn") and where the plural
is the same as the singular. I don't think this is in keeping with the
spirit of the question.
There are a number of Italian words where the final "o" is dropped to
form a plural:
Other words are:
hemiepes n pl hemiepe
:a dactylic tripody having a spondaic third foot or lacking the two
short syllables of the third foot
koruna n pl korun or koruny also korunas
:the basic monetary unit of Czechoslovakia
microstomia also microstomus n pl microstomias also microstomi
:an abnormally small mouth
necropolis n pl necropolises or necropoles also necropoleis or necropoli
:a large elaborate cemetery of an ancient city
samanid n pl samanids or samani
:a member of a 9th and 10th century Persian dynasty ruling from Bokhara
and encouraging literature and art
volksdeutscher n pl volksdeutsche
:a person of German ethnic origin long settled in a central or east
European country, repatriated for political reasons by the Nazi regime,
and expelled into West Germany after World II
Unless noted otherwise, all words occur in Webster's Third New International
Dictionary, Merriam-Webster, Springfield, MA, 1961.