Straight Dope, man. What a column. And here it illustrates, as always, that English is a very weird language:
Penis is a third declension noun, not second declension. These nouns often end in -is in the singular and -es in the plural. The English style -ises is sometimes preferred. Hence, we have penises (half of us do, anyway), and mantises and pelvises, but only more rarely do you see penes, mantes, and pelves, though they are not incorrect. In many cases, only the Latin form is acceptable: We have testes (some more than others) and crises and psychoses, but never testises, crisises, or pyschosises.
I always get annoyed when people say “forums” instead of the obviously more correct “fora.” But I’ve had to give in: “fora” is, frankly, pretentious, and therefore is perfect for this site, but it doesn’t fit on something like, say, Go-Mono, which actually does have fora. Or forums. A discussion board, anyway.
Secretly Ironic 