Literature professor with training in classical and medieval Latin here. I would have to vote for "Jaraxxuses" on linguistic grounds, and I'll explain why below.

The reason why so many English nouns that end in "-us" are pluralized with an "-i" is because most of them originated as Latin words of the second declension, which take "-i" for the nominative plural. As one of the above posters points out, however, many other Latin nouns -- we call these fourth declension nouns -- also end in "-us" but are instead made plural with a long rather than short "u," rendered "-ūs." The proper way to pluralize these nouns in English, however, is simply with "-uses" ("omnibuses," and so on). When English speakers intuitively attempt to pluralize a word such as "Jaraxxus" (which, although obviously pseudo-Latinate despite the double "x," is not in fact a Latin noun of ANY declension) using an "-i" assuming that it must be a second declension Latin noun imported into English, we refer to this linguistic overcompensation by the term "hypercorrection," because actually the "-i" rule doesn't apply here. Even so, hypercorrections can become recognized parts of language themselves: "octopus" is not a Latin noun, either, and yet most dictionaries will give you "octopi" and "octopuses" as correct plurals in English, just because it's been treated as if it were for so long by English speakers.

For example, to this day, we academics endlessly argue with each other about the proper way to spell the plural of the word "syllabus." Although you will get pedants telling you to say "syllabūs" and treat it as a Latin word that hasn't been Anglicized, I use both "syllabi" and "syllabuses" interchangeably myself; after all, when you teach medieval literature for a living, you necessarily adopt a more descriptivist than prescriptivist approach to language. As I tell my students, the idea of a correctly spelled word is a relatively recent invention in the history of the English language... just read some Chaucer!

But, in short, it has nothing to do with proper nouns, as another poster claims. In Latin, a proper noun that was a second declension noun would indeed be pluralized with an "-i" (this was in fact a frequent occurrence for names of families, tribes, and other groups referred to in the plural like we might say "the Americans" or "the Smiths").