Warboars are hyper-aggressive wild hogs native to the Games Workshop multiverse. First appearing in Warhammer Fantasy as the heavy cavalry of the Orcs & Goblins, they subsequently migrated to the universe of the 41st millennium with the release of Rogue Trader, as this first edition of Warhammer 40,000 was predominantly based around the notion of "Warhammer Fantasy In SPACE!" They've since migrated to Warhammer: Age of Sigmar as well.
As the editions rolled past and 40K tried to tone down both the "fantasy" aspect of its Space Fantasy roots and its direct connections to Warhammer Fantasy, the 40K version of the warboar slipped into obscurity. Still, /tg/ remembers...
Orks (or Orcs) that ride warboars are known as Boarboyz.
For reasons unbeknownst to anybody, the forests and hills of the Old World are infested with huge herds of particularly large and savage swine. It's possible that the answer may lie in a combination of their porcine resilience and their omnivorous diets; warboars may descend from herds of wild pigs that have fed on the flesh of dead beastmen or other Chaos beasts left to rot in the wilderness, and have managed to gain in size, strength and ferocity without picking up any more major or detrimental mutations. This same process may ultimately have culminated in the birth of the Razorgors, which likely descend from swine that either glutted themselves on a particularly beastman-heavy diet or which were less resistant to mutation.
Warboars are still omnivorous, but definitely prefer meat to vegetation. Even by the standards of normal wild swine, they are savage, vicious, ill-tempered brutes - all traits that the orcs admire greatly. Warboars grow bigger than horses, able to carry an orc in full battle regalia upon their backs, and will attack just about anything in pursuit of food or just for the sheer hell of it. Like normal swine, warboars have natural armor under their hides and dulled senses of pain, allowing them to shrug off terrible injuries and continue to fight on even when mortally wounded.
Because of all of these traits, orcs have universally adopted warboars as their steeds of choice - well, except for that bunch of weirdos over in Bretonnia who have taken to riding hagranyms instead. The tradition of riding a bloodthirsty warboar into battle is so ancient that even the Stone Age, technophobic Savage Orcs gleefully ride them into the fray.
Squigs aren't the only type of animal that seem to be cultivated and used by the Orks. Somehow, in the 41st Millennium, the Orks managed to tame and breed a large variety of boars called the Warboar. These animals are obviously not part of the Orkoid ecosystem, yet they have found a special place in Ork society.
These Warboars (or "Gruntas" in Ork Language) are large, brutish and intemperate animals, closely resembling the wild boars of ancient Terra, although grossly exaggerated in size. Where the Orks found these boars, no one knows, although most in /tg/ would estimate that the size of these boars is gonna create some humongous bacon... hmmmm... no wonder the god damned Orks have these things everywhere. Even in the 41st Millennium, Humans and Xenos can agree that nothing still tops bacon as the number one meat source. They are sometimes used by Ork warbands, particularly by Snakebites and Feral Orks.
Throughout all of Ork history, Warboars have been used as steeds, because of their ferocious temperament and prolific abundance throughout the galaxy. Stout and covered with coarse, bristly hair, the Ork race has long had an affinity with the creatures because of their common violent and intemperate natures. Warboars are favoured as they possess the toughness that faster steeds lack.
In the current Age of the Imperium, the relatively advanced Ork Clans shun the primitive warboar in favour of bikes and other vehicles, and warboars are now seen only in Feral Ork tribes. Their induction into a tribe usually takes place when an Ork (often a Trappa) is forced to prove how tough he is by catching one. If the Ork wishes to prove he is particularly powerful, he will attempt to 'train' the warboar for riding. Training consists mainly of brutally breaking the animal in. The Ork benefits from the speed and ferocity of the creature and the boar for his part in the bargain is treated to a daily supply of gruel, a smelly sty, and the occasional smack across its snout with a large stick. Riders of these boars are called Boarboyz.
There are two distinct variants of the normal Warboar.
The best boars are chosen by the Meks and Doks and heavily augmented with crude weaponry and armor. Sometimes this augmentation is so heavy that the Warboar can no longer be considered simply a boar, and it is then called a Cyboar. Cyboars often require a lot of resources to produce and so are usually reserved for important Nobs and Warbosses. Improvements could include such things as replacing the boar's tusks with even more vicious metal ones, giving it powerful, hydraulic or piston-driven limbs, or reinforcing its hide with metal plating. One common feature among cyboars is a large cluster of stimulant injectors to increase the animal's speed (often operated by a large red button).
Obviously, these forms of surgery and repairs can take up a lot of the Mad Dok's time if the tribe contains a large number of boars. Because of this, Mad Dok's in Feral Ork tribes often charge a hefty fee so that his work won't get swamped with Feral Orks trying to pimp their ride.
The basic Cyboar leveled up.
Sometimes a Warboss will demand that a Mad Dok create an even faster steed more befitting of their status. In this case the final product will often be referred to as a Super-Cyboar. Its more heavily cyborged body offers even greater protection when ridden. Super-Cyboars are more machine than pork.
Super-Cyboars are usually selected from the largest and most ferocious of a tribe's Warboars, and is almost always ridden by a Warboss. These mechanical creatures are fitted with layers of heavy armour plating, powerful bionic limbs, and razor sharp metal tusks. Each Super-Cyboar is equipped with a device commonly known as the "Da Big Red Knob". This button, when pressed, will flood the Super-Cyboar's body with stimms, causing it to hurtle forwards at reckless speeds. The Warboss will have this system linked up with all nearby Cyboars, causing them all to rush into a headlong charge with the simple press of a button.