In general words which have been around long enough acquire an "es" after a consonant in order to make a plural - this also applies after the x ending. In the long slow transition from Old through Middle to Modern English, the "e" has been replaced by an apostrophe, this is oftentimes the case with the possessive form.
Plurals and possessives should be considered separately for the sake of clarity.
With regards Xerox, being a brand name and a recent one at that, may be exempt from Old English spelling rules. The consensus in a search engine query, the plural seems to be Xeroxes like boxes which is also recent enough addition to have not dropped an "e", or the "s" for the sake of phonetic redundancy. The respective plural genitives would be Xeroxes' toner cartridge supply, and boxes' labels.
The plural of ox is oxen, so the oxen (or more cringe-worthily oxen's) pens are secure. Singular the ox' pen is secure. The ox's or oxens
pen is secure.