I have two thoughts. First, why not take ten seconds to look it up? Various online dictionaries list two plurals for penis: penises and penes. No penii.
But then again, since we’re writing in English, why use a foreign-language plural at all? There’s some odd compulsion to mangle plurals of words that seem Latinate – penis and Prius being the prime offenders – and it’s just not necessary. Priuses. Penises. Octopuses. Embrace them. Not literally.
Bonus peeve: plurals never require apostrophes.
(Okay, there are some edge cases. “Do’s and Don’ts” is a nightmare. But “hot dog’s”? What is wrong with people?)
