Tyranid Harpy
The Tyranid Harpy is a large winged species of Tyranid comparable to a winged Trygon. They appear at the initial stages of Tyranid attacks working together with the similarly winged Gargoyles and other flying individuals of the Tyranid host. Unlike the Crone, which focuses on air superiority, the Harpy pulls double-duty as a combination bomber and assault craft.
A Harpy has a long and sinuous body with massive leathery wings and a broad, armoured head. To better harness thermal currents, its body is hollow, resulting in it being relatively fragile compared to Tyranid creatures of similar size. However it has superior agility in the air than even technological fighter-craft can manage.
The Harpies' undersides are covered in bloated cysts that can rain down Spore Mines onto luckless prey. It can also emit an ear-splitting shriek that might rupture eardrums and is excruciatingly painful or even fatal, and at the very least causes disorientation. It may also be given twin-linked Stranglethorn Cannons, Heavy Venom Cannons, equipped according to the tactical needs of the Hive Fleet. Opportunistic by nature, Harpies avoid protracted assaults, preferring instead strafing runs and swooping dives, or engaging enemy skimmers ill-suited to fighting back. Their shriek is what gave them their name and their relation to the Trygons is a highly argued topic by the Magos Biologis of the Adeptus Mechanicus. They may also possibly be related to Hive Tyrants and/or Harridans.
On the Tabletop
The Harpy is a quick, well-armed addition to a Tyranid army, but it won't win games on its own. Instead, the harpy serves well as a support unit, exploiting targets of opportunity with its ranged weapons or piling into a combat to help swing the fight to your advantage.
With a speed of 30" base and access to the full suite of Tyranid movement shenanigans, the Harpy is easily one of the most mobile units in the army, if not all of 40K. This makes it ideal for harassing isolated units or targeting hard to hit flyers that think themselves safe from melee.
Speaking of melee, the Harpy is merely ok. Its scything wings allow it to threaten infantry or light vehicles, but with a toughness of 6, 12 wounds, and a 4+ save, it will fold in the face of a dedicated close combat specialist. Notably, Its sonic screech forces an enemy to fight last after every other unit has fought, but this won't matter if the Harpy is the first one doing the charging. Where it does shine, however, is when the Harpy jumps into an existing fight and buys another round for your assault specialists. Not to mention, it can simply fly away from any combat it doesn't want to get stuck in.
In terms of Shooting, the harpy packs a considerable amount of firepower into its frame. Notably, it is the only monstrous creature in the game that can take two heavy weapons. Not even the Carnifex or Hive Tyrant can claim that right. Its starts with two Stranglethorn cannons, making it ideal for thinning out large units before your ground forces can assault. It can swap these for two Heavy Venom Cannons and go light vehicle hunting. The high strength and 3 damage of the HVC make them a serious threat to light vehicles. If the Harpy has one weakness, it is its mediocre ballistic skill of 4+. This means any negative modifiers at all will drastically reduce its efficiency.
Finally, and of special note, is its spore cysts. the Harpy can drop spore mines on a unit it moves over during the movement phase, and potentially either deal mortal wounds or spawn a spore mine nearby. roll one dice for each model in the unit up to 3. on a 4+, the mine hits. You then roll to see if it explodes. on 1, nothing happens. on a 2-5, you cause 1 mortal wound. on a 6, you cause d3. If it fails to hit, you can place a spore mine model within 6" of the enemy if it fits. Notably, you can use this ability on an enemy unit locked in close combat. Apparently, Spore mines are smart enough not to explode on top of their fellow Tyranids.
Battlefleet Gothic Armada 1 & 2
Tyranid Harpies are, comparatively, the WORST fighters in the game. This is not to say they are bad per se, as most of their stats are actually quite average. The problem is that they are the slowest fighters in the game. As such, it is much better to use your Harpies to bodyguard important ships rather than going all out against enemy attack craft.