Spore Mine
A Spore Mine is a living, floating Tyrannic creation which will explore if any non-Tyranid creatures gets too close to it. They are created by Biovores, who gestated the mines deep into their bodies, who them launch the spore with a powerful muscular spasm from the long tube on their bodies...and if you are feeling uncomfortable reading this trust me you should.
The mines float through a gas bladder shake and they drift slowly alone, minding their own business until any non-Tyranid without a brain gets too close. Of course this means any random rabbit could set these things off but then what do you expect from the intelligence of cosmic bugs? They have been known to drift for months or even years before they are set off.
They are said to possess a low animal intelligence despite not having a head and can feel heat and vibrations through feelers on their bodies. They are full of all sorts of delightful poisons and toxins that would make a Slaanesh worshipper swoon with delight and capturing them as a delicacy for a Keeper of Secrets is said to bring high rewards.
They come in three fun varieties: Poison Spore Mines (full of poison duh); Frag Spore Mines (which have lots of sharp shards in them and explode like a grenade, don't fix what ain't broke right?); and Bio-Acid Spore Mines (which spray stinky acid everywhere and well, you can guess what happens next).