The Swarmlord is a unique synapse creature of the Tyranids.
Official Canon Edit
The Swarmlord is unique because it has a distinct personality, and every time it is killed in battle, its memories are preserved by the Hivemind and placed in a new body. Thus the Swarmlord always learns from experience and can be found across different hive fleets.
The Swarmlord was first spotted during the invasion of Macragge, having fought and won against Marneus Calgar by removing his limbs.
TTS Canon Edit
Calgar's duel with the Swarmlord ended with him ineffectually charging and kicking at the Tyranid with only one leg. Confused by his stubborness, the Swarmlord simply left Calgar as a stump and moved on to the rest of the battle.
The Swarmlord appeared again when the remnants of hive fleet Behemoth attacked a Hive City that the Deathwatch and Kryptman were attempting to defend. Before it could finish off the defenders, the Ultramarines appeared to drive out the Tyranids, and Calgar personally challenged the Swarmlord to a rematch. Calgar effortlessly curbstomped the beast, complaining that the fight was a waste of his time.