Some nouns of foreign origin form their plural like in the source language. These plural forms do not correspond to the German inflectional system. They are normally only used in technical terminology..
Typical examples are:
Nouns on -um
Nouns on -on
(Italian) nouns on -o
Words of English, French or of other origin, which form the plural with -s are not included in this group. Since s exists as a plural ending in German, too, these words are classified into the corresponding general inflection classes (e.g. Evergreen, Gourmet).