Tyranids have an interesting collection of names for their creatures. Here are some of their meanings. I have classified them in a couple of groupings. Italicized choices come from Epic, Forgeworld, or Armourcast.
The Female Ones
Termagant: Means a harsh-tempered or overbearing woman.
Harridan: One of my favorite words, it means a strict, bossy, or belligerent old woman.
Dominatrix: A dominating woman, it has other meanings but I'm sure you already know those.
The Greekish Ones
Biovore: Literally, life-eater. Methinks the technical Greek is Biophage, but the -vore is Latin, so it's one of those fused words.
Malanthrope: Another fused word, Mal- being the Latin for bad, and -anthrope being human being.
Hierophant: A priest who interprets sacred mysteries.
Hierodule: A slave in an ancient Greek temple.
Dactylis: Comes from the Greek meaning "fingers" if I recall.
Exocrine: Meaning an external secretion.
Zoanthrope: A portmanteau of zoo- meaning animal, and -anthrope meaning man. This term was coined by Gene Wolfe in his Torturer series (he also uses the terms Lictor and Carnifex heavily in that series, the books were an influence on GW).
Hormagaunt: I'm just guessing on this one, but horman is the Greek for "to set in motion" (it's where the word hormone comes from), so this would mean a gaunt that is set in motion. A good name for a fast moving creature.
The Latin Ones
Carnifex: Means butcher. For the plural, just use Carnifexes. If you really want to get all Latin, you can say Carnifices. However, there are several versions of Latin plurals and I don't want to recall my 4th grade Latin lessons so just use the -es ending. Similarly, the plural of codex is codices.
Edit: I stand corrected- The original sources I looked up said it meant butcher, and I assumed it to be correct because the roots would mean flesh-maker, but Elazar pointed out otherwise. I did some further research and he indeed is correct, a Carnifex is an executioner.
Malefactor: Means evil-doer.
Lictor: A lictor was a civil servant in ancient Rome, who acted as bodyguards of sorts. It means "one who binds." Interestingly enough, the bundle of rods carried by a Lictor was called fasces which is where the term "Fascism" originates.
Haruspex: A religious official who divined the future by looking at the entrails of animals.
This list isn't comprehensive. I figured I'd point out where the words for our more oddly named creatures come from.