==> language/english/plurals/drop.terminal.s <==
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.