Zoanthrope
The Zoanthrope is a Tyranid species characterised by its massive head, a long thin body dangling under it and having enough psychic power to turn a Land Raider inside out.
Physiology
Prior to third edition, Zoanthropes looked like giant two-legged Zoats. Yes, this was utterly stupid and was retcon'd with extreme prejudice.
The Zoanthrope is a tall, slender creature with an oversized cranium that makes up more than half its mass and about half its body length. The Zoanthrope's head is far too large for its body: if it were a natural creature its would snap under the weight of the noggin. But the Tyranids do not obey the laws of nature: the massive brain housed by the head serves as a relay for the massive psychic power of the Hive Mind, giving the Zoanthrope awesome psychic powers. What powers you ask? How about the power of flight? That do anything for ya? That's levitation, Holmes. How about the power to kill a yak, from 200 yards away... with mind bullets! That's telekinesis, Kyle. How about the power... to move you?
- ahem*
Sorry for that. But Tenacious D did all but perfectly describe the Zoanthrope's abilities in their song Wonderboy. With their powers they can float (but not that high off of the ground), they can shoot holes the size of a Predator Tank in a Predator Tank (or melt small groups of MEQs), and have the power to move you. With "you" meaning "your creatures that have Instinctive Behavior" because the Zoanthrope is a Synapse Creature. It also has a strong forcefield surrounding it, meaning that it has a good chance to shrug off otherwise lethal shots. And in older editions there were almost as good as Warriors at fighting. Like any Psyker, the Zoanthrope is vulnerable to brain overload and subsequent exploding craniums (or as Orks call it; 'eadbanging). If this happens the creature's brains burn out and it falls to the ground like a grotesque deflating balloon.
One Zoanthrope called "The Doom of Malan'tai" evolved to feed off of psychic energy. It got the name after singlehandedly destroying Craftworld Malan'tai when it drained the Infinity Circuit. Not anymore, this is just some completely nondescript Tyranid that is in no way a Zoanthrope and has never been because that would imply that someone can make a model for him and not have to buy from GW.
On the tabletop
Zoanthropes are used best against either vehicles or big monsters. As a Brotherhood of Psykers at Psychic Level 2 they have access to two powers: the tank-wrecking Warp Blast/Lance and one power that you roll for. The downside to this is that your psychic shooting attack is Mastery Level 2, which means if you use one of your powers you cannot use the other. And their Brotherhood of Psykers rules means that you only need one test (and your opponent gets one Deny the Witch roll) to use your powers: Warp Blast/Lance is improved by 1 for each Zoanthrope in the brood, so with three of them your shots become Assault 3. This is good because it speeds up rolling for your shots, but one lucky roll from your opponent wastes an entire turn (and maybe your unit as well, if you're unlucky).
Because of the short range of their powers they have to come dangerously close to both their target. This is why you should either keep them close to Venomthropes or provide enough distractions so they are not shot to death in one turn. It used to be that you could put them in mycetic spores and Deep Strike them behind whatever you wanted gone, but the 6th edition codex lacks mycetic spores entirely. And while they have a 3+ invulnerable save they still have a 1/3 chance to die to a single Krak Missile. Neither should you forget that they can be tied up in combat if they get charged: they are not very good at fighting and will stick in combat longer than you'd like. As they are Synapse Creatures, they will keep your Gaunts controllable, which is always nice but again remember the range on their weapons and that they are not as fast as Hormagaunts. And while a single shot is likely to deal great damage to a target is is not guaranteed it will be destroyed: as such it is smart to take several of these in a single brood.
In Last Stand
In Last Stand, these things are assholes: Giant, flaming assholes. They do enormous damage, have immense knockback, a huge area-of-effect, and are remarkably durable for a floating snake-brain thing thanks to a Warp Field that acts like an Iron Halo. They are the bane of every Lord General, since they can effectively one-shot the entire minion squad a General may have with him. They are, thankfully, easy prey for Sniper Rifle rounds and Shotgun solid slugs, and are a perfect target for a Chaos Sorcerer who is has the Improved Summoning wargear to clone.
Maleceptor
In late October 2014 GeeDubs made a token attempt to help the Tyranids feel less useless and released two more monstrous creatures: the Toxicrene and the Maleceptor. The latter of which is described as just being an extra large Zoanthrope (which is why it's in this article right now instead of its own) with extra armor and some legs because apparently bigger brains put out less levitation. The two MCs appear to share the same model pack, with the Maleceptor growing six exposed brains out of its back.
Anyway, while the Toxicrene was at least decent, the Maleceptor became incredibly unpopular within minutes, clocking in at 205 points with only 4+/5++ and Mastery Level 2. Its big gimmick is a unique power called Psychic Overload, and can cast it on three different targets a turn. It brings back memories of old Doomy Malantai, what with target takes a Ld test on 3d6 or else take d3 wounds with AP2 Ignores Cover. Only problem: it's Warp Charge 2. Go ahead and do some test rolls to see how many warp charge dice you need to make three WC2 powers. Also it's a Focused Witchfire power, so most opponents will force you to roll to-hit; did I mention the Maleceptor is only BS3? White Dwarf plays it up as the Tyranids' "psychic sniper", and it's so hilariously inept at it that you wonder how the Hive Mind is still a threat at this point.