‘Penis’ derives from ‘pēnis’ – a third declension Latin noun for tail (or, even, the male member) – so, true to the rules, could be pluralized as ‘pēnes’. As it has been adopted into English, native rules also can apply making it ‘penises’.
Never ‘penii’.
From Robert at the Marmot’s Hole, we get a hotchpotch of big cats roaming 19/20th Century Korean gold-mines; portents of doom for the reign of King Hyegong in 7th Century Korean during the Silla period, as well as his father, King Billy Gyeongdeok’s seven foot willy.