Hormagaunt
Hormagaunts are the Tyranids' basic assault organism (read: tarpit). Equipped with a pair of Scything Talons, their job is to rush into close combat and either slice things into nice small pieces, or keep enemy walkers from walking anywhere. They can be assisted in the former role by the addition of Toxin Sacs or Adrenal Glands, making them even more killy. They can be assisted in the latter option by keeping some Synapse nearby. As they're members of the gaunt genus, they're unsurprisingly most effective when fielded in numbers, though they don't need quite the same numbers termagants do. Good for drowning your enemies in poisoned attacks.
Despite wanting to be run in large groups, gaunts are juuuust a bit too costly and too fragile to serve as true horde-wave forces. Upgraded, they have a similar cost and stat line compared to daemonettes but without the nice rending, fear, or invulnerable save. They are probably best used as a support unit to back a larger push or to block for a specific unit.
Their Role for the 'Nids
Before the full attack of a Tyranid Hive Fleet, Hormagaunts are often dropped in Mycetic Spores in such large numbers that even if many are destroyed by orbital defences those that make planetfall will pose a serious health problem to any inhabitants.
Hormagaunts have a deadly instinctual ability to hunt that needs little direction from the Tyranid Hive Mind. They are so single-minded in hunting that they will ignore personal injury and tiredness until they have run down their quarry. Hormagaunts will then feed upon their prey, ripping pieces of flesh apart with razor sharp teeth. However their extremely fast metabolism will always drive them to constantly seek out new victims. They are usually left to their own devices once a Tyranid attack is underway, though occasionally the Hive Mind will intervene and direct them towards a more distant and more important target.
Hormagaunts, unlike other Tyranid creatures, are female hermaphrodites and have the ability to lay hundreds of fertile eggs in the ground and so reproduce independently of a Hive Fleet. Each Hormagaunt has a short lifespan but once one generation has died a fresh wave will have hatched and grown to maturity, ready to carry on ravaging a world where the previous generation left off. So like ants or Orks, clearing a Hormagaunt is a colossal pain in the ass and may require sustained orbital bombardment filled to the brim with Prometheum. Otherwise the surviving population will have a new sport or training program for the Planetary Defence Force.