Ok, so I have spent the last... day looking for a solution to this and I could not reach a serious conclusion: What is the plural of Krampus? First of all, Krampus is not a Latin word. Therefore, the idea that its plural would be Krampi fails right there. Only words from the Latin, in the English language, could have its singular -us and plural -i.
Second, best I've reached so far was: it originated somewhere in the Indo-European family, probably of the Germanic Branch (or if you prefer, Proto-Germanic origins), now which part of it... that's a mystery. Some people say it's of the Old High German, but looking closer at the possibilities, I have found no examples of words ending in -us, and -en (Krampen) could be possible since Krampen would be the plural form of claw, meaning the radical is Kramp. (It was falsely reported at Wikipedia that it was a Norse Word. I researched it and there is no evidence of it being a Norse Word. In fact, the very article which was used as reference that it was a Norse Word in fact said it was a German Word!)
If you do have a book on Old High German, on Old Norse, or some other language which is reliable enough and that could give the following information it would be very much appreciated: Structure of the Words and Syllables (What possible letters/phonemes can be present on which position of any valid word) of any of the Old High German, Norse Language, Contemporary German Language branches, Morphological and Phonological processes which were involved in the transition of Old High German to any other German, as well as the processes which took place in the separation of the Norse Language from the Proto-German.
Summering it up, Krampus is not a Latin word, it is of the German family of language. Which one I could not find, but as far as Nouns go, it's either Krampusen or Krampen. Krampusen because foreign words have -en added to then when they're made plural (ok, here let me say that we're talking about NOMINATIVE declension. If we were to venture into the other declensions then we'd have a serious problem, one which would make no sense at all in English) but we could also have Krampusata, if we were to use the present German as the ruling case. Krampen/Krampusen or Krampata/Krampusata. These are the best cases which I have been able to reach.
P.S: Yeah... I did get bugged off when I saw the plural of Krampus as Krampi.