Neuroloid
Neuroloids are strange looking Tyranid organisms that look like they are related to other floating Tyranid jellyfish organisms like the Spore Mines and the Meiotic Spores. Although some in /tg/ would just like to refer them as purple Flying Spaghetti Monsters.
Overview
Neuroloids form a symbiotic relationship with the larger Neurotyrants. They seem to act more like the Neurotyrant's own girl posse rather than actual bodyguards; assisting the Neurotyrants latent psychic abilities.
A Neurotyrant always have two Neuroloids following it closely around. That's all the fluff there is to say about these things.
Crunch
Crunchwise on 10th Edition, Neuroloids aren't a unit per se and more like support equipment. Neuroloids are to be uses as tokens for the Neurotyrant's Synaptic Relay ability. Pretty nifty.