<pre>
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.
</pre>
