The Tyranids are an extragalactic race of insect-like aliens that exist only to consume all consumable things in order to reproduce and evolve.
Tyranids are one of, possibly even, the greatest threat to the inhabitants of the Warhammer universe. They consume entire planets and once they have finished all that is left is a barren, airless rock where it is impossible to comprehend that life once existed on this now Emperor-forsaken planet.
The bioengineering capabilities of the Tyranid race far exceeds that of any known race. Tyranids will often develop new strains of creatures in order to combat a single threat. It is highly unlikely that all the strains of Tyranids have been witnessed by Imperial forces.
Breeding an almost endless swarm of lesser Tyranids capable of intelligent thought would be incredibly taxing to the biomass levels of the Tyranid swarms. To counter this problem all lesser Tyranids are developed with only a very basic level of intelligence and are instead led by larger creatures. The creatures are tasked with instructing their lesser brethren through psychic synapses. A large force of these creatures will always be present in any Tyranid invasion much like the Imperial guard might field a large number of commissars, sergeants or another higher ranking officer. Imperial forces have, however learned to 'shoot the big ones' in an attempt to strip the Tyranid horde of its leadership and through this, its intelligence.