I've been having a terrible allergy season. That, and maybe just a little too much They Might Be Giants, caused me to research the word miasma (as in a cloud of ragweed that follows you around) for a possible Word of the Day post. Here's what I found, dictionary-wise.
n. pl. mi·as·mas or mi·as·ma·ta
1. A noxious atmosphere or influence: "The family affection, the family expectations, seemed to permeate the atmosphere . . . like a coiling miasma" (Louis Auchincloss).
a. A poisonous atmosphere formerly thought to rise from swamps and putrid matter and cause disease.
b. A thick vaporous atmosphere or emanation: wreathed in a miasma of cigarette smoke.
See the second plural, miasmata? Now, that's my kind of irregular plural. Or, as it turns out, a perfectly normal plural for Greek, borrowed into English for this juicy word. You've probably heard of stigmata but might not have realized that it's the plural of stigma.
Certainly we're not obligated to use all the original declensions of a loan word, but it can be fun. Via the all-knowing wikipedia, "Final ma in nouns of Greek origin can become -mata, although -s is usually also acceptable, and in many cases more common."
Other English words of Greek origin that could take the -mata (-ματα) plural include schema (schemata), dogma (dogmata, but isn't the point that there's just one?), and lemma (lemmata) which bring up all kinds of additional dilemmata, don't you think?