Exocrine

A huge living weapon with a carrier beast is what you could ably sum the Exocrine to be. This creature is actually two Tyranids in one: The huge gun on it's back which can fire a huge ball of plasma or several small streams; and the Exocrine itself which is just a mule to carry the gun and protect it.

The gun itself controls the exocrine and can only fully concentrate on targeting and firing when it mentally smacks the dull creature to stop moving about. This has lead many Imperial observers to believe the gun is actually more intelligent then it's carrier and endless lolz in the fanbase as the completely sensible question of why an intelligent, evil overmind like the Hive Mind would create such a weapon with the drawback of being unable to fire if the carrier even twitches a little. Other less bio-morph weapons seem to not have the trouble of a mindless oaf to worry about after all.

Regardless the Exocrine is the most feared of the ranged firepower creatures in the Tyrannic army, which though is not a compliment for a race renowned for poor firepower, on par with the Orks and their wild chaotic shooting sprees.