"Platypus" or "platypuses" are both accepted plural forms.
The plural of platypus is not platypi. It is unfortunate that some dictionaries include platypi as a plural. Listing "octopi" as the plural of octopus is also incorrect, for the same reason. Both words are from Greek, not Latin.
"Platypi" is a colloquialism, and incorrect both grammatically and etymologically. The term uses pseudo-Latin rules.