No, it is Krampuses. Why? Because the word is German (The German plural is Krampusse but is considered non-standard in English). In general you only use the -i plural form for words that are Latin derived and originally masculine second declensions ending in -us.
Words that you are likely to use and are pluralized with -i:
Alumnus
Radius
Cactus
Not much else.
Words that you are wrong, or at least using a less preferred form if you attempt to pluralize with -i
Octopus (comes from Greek, not Latin, and is correctly octopuses)
Virus (Long-sh grammar explanation but short version is if you said “virii” to a Roman they’d be all like WTF?)
Platypus (same as Octopus)
Anus, Campus (Just because ani and campi sound really odd)
Pretty much everything else.
Freedumbring 