Tyrant Guard
The Tyrant Guard is a species of Tyranid specifically engineered to A: guard Hive Tyrants (hence the name) and B: be tough as balls.
Physiology
The Tyrant Guard is approximately one and a half times the size of a human, and half as tall as a Hive Tyrant. Though their actual body lengh is much taller, they stand on all fours in a pose similair to that of gorillas or overgrown guard dogs. Which is pretty much their role on the battlefield: to stay close to their master and eat anything that gets close, be it assailant or heavy weapons fire. They are living shields, their sole purpose in existence being to soak up heavy weapons fire and die so that their Hive Tyrant does not. For this purpose they are designed to be exceptionally durable and well-armored: they can all but ignore small-arms fire and even when hit by a dedicated anti-tank weapon they will just not stop. Its large armor plates cover most of its body to negate any weak spots: it does not even have eyes so there is no weakness for the enemy to exploit. It takes continuous, accurate fire to take down a Tyrant Guard before it and its charge can reach their target, which often means that if you do manage to kill it, there are a few dozen Hormagaunts snacking on your jollies. And despite this they have fewer wounds than a warrior and only a 3+ save, and no FNP because 'Nids have been criminally underpowered for three editions and Cruddace keeps making shit worse.
The Tyrant Guard is an extention of its charge's will: if the Hive Tyrant it is linked with is slain the Tyrant Guard flips its shit and will start to MURDER EVERYING IT CAN GET NEAR. Not out of revenge or to martyr itself, but for the simple reason that the Tyrant Guard is a creature whose intelligence is on par with that of a Ripper. This does not mean that the Tyrant Guard is a harmless foe: its sheer bulk and massive claws means one can punch you into meatbread in a single blow. Tyrant Guard can be armed with scything talons, lash whips or boneswords, so they can deal with a variety of foes in close combat.
Because of their small size in comparison to the Hive Tyrant and their limited potential to move their arms upwards, the only logical way Tyrant Guard could stop incoming shots would be to jump like Super Mario. It is rumored that the Tyrant Guard have had their physiology created through the DNA the Hive Fleet gained from defeating Chapters of Space Marines, but this is considered to be heretical thinking and will be punished by blamming.
On the tabletop
Tyrant Guard can only be taken in tandem with a Hive Tyrant, but they do not take up an HQ slot. They can be taken in groups of 1-3 at a decent price for good toughness and fighting ability. If you do not take a Flyrant they are mandatory: a footslogging Hive Tyrant or Swarmlord will be eaten alive by any army with decent firepower if not protected. Their standard secondary weapons are Scything Talons: if you can spare the points it is advisable to either go with the Lash Whips (when dealing with armies that have lots of attacks) and Bonesword. Overall, if you can take them you really should consider doing so if you do not want to lose your Hive Tyrant on turn 2 to mass Railgun fire.
TL;DR 2 extra wounds for your hive tyrant for each one.
Notably, the Tyrant Guard changes appearences radically every time it gets a new model. At first it looked like organic bug-romans, then it had a 3e makeover of aforementioned bug-romans, then it became a murderous pill bug shown in the big black and white picture to the right that is featured in Dawn of War 2, and now it's got flaring pauldrons (thus making it more obvious they are derivatives of space marines ), is substantially skinner as a whole, and taller and holds it's weapons in a completely different way. Compare this to other Tyranid models who have largely looked the same since the 2e-3e switch (with the exception of the Tyrant who went from a huge warrior to what he is now) with some rather minor artistic shifts as GW gets better at putting in finer details and goes through different phases of aesthetics it wants to show off.