Tervigon
A Tervigon is very similar to a Tyrannofex, but it's more melee-inclined and instead of having a bio-weapon has spread legs and a uterus filled with as many egg sacs as your opponents have bullets. In combat, she births entire broods of Gaunts, allowing you to overwhelm enemy groups with sheer numbers, since she puts out 3d6 Termagaunts a turn - and can do this every shooting phase. If you rolled doubles, she couldn't produce any more Gaunts for the rest of the game, but otherwise the skies are the limit. Though she's not quite as directly useful as other Monstrous Creatures the simple fact that she's essentially the Commander Kubrik Chenkov of the Tyranids means she's a massive source of tarpitting. She's also good in close-combat, being a Monstrous Creature and all, but there are other Tyranid organisms that do it better.
The Tervigon's constant Gaunt production has resulted in many hilarious discussions on /tg/. Many 'Nid-playing fa/tg/uys asked how her ovaries came to be fertilized and are ready to spew forth meat shields for the hive mind. Some think she is akin to an aphid, being born pregnant (the anti-lulz strictly-canon brigade leans toward this, since Tervigons act as anti-boarding parties during FTL flights); other wilder claims support her in a relationship with the local Hive Tyrant while yet others say she is a whore hiring herself out to Trygon, Mawlocs, and when she's desperate, Haruspexes and Maleceptors. The most common claim is that she is also a he. In any case, the Tervigon is one of the few all-female units in 40k, and one of the better ones to boot.
Fittingly enough, in 7th ed we're running into skub-tastic flame wars about whether 'gaunts spawned from battle-forged Tervigons get any of the formation bonuses (namely Objective Secured), which is kind of relevant because Gaunts are also scoring units. A recent FAQ states that they do, strangely enough, despite the consensus being the opposite. Neat.