Q: What words have their plurals formed by dropping the final letter?
A:
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
- arpeggio capriccio carroccio fascio impresario intarsio latticinio pasticcio preludio scenario seraglio solfeggio
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.