The suffix -i is one of several suffixes in the English language that form foreign plurals (-a and -ae are two others). Many nouns derived from a foreign language retain their foreign plural, at least when they first enter English, and particularly if they belong to a specialist field. Over time, though, it is quite normal for a word in general use to acquire a regular English plural. This may coexist with the foreign plural (e.g., cactus, plural cacti or cactuses), or it may actually oust a foreign plural. Note that not all Latin words ending in -us have a plural that ends in -i: for example, the Latin word apparatus is unchanged in the plural.
From Semitic and Indo-Iranian adjectival endings.
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