The Beast is the name given to one of the largest and most powerful Ork warbosses in the history of the Warhammer 40,000 universe. Not to be confused with the Beast of Armageddon, Ghazghkull Mag Uruk Thraka, the Beast was active during the 32nd Millenium - roughly a thousand years after the Horus Heresy had come to a close -, leading an Ork WAAAGH! of apocalyptic magnitude. It devastated the Imperium of Man at a level not seen since Horus' betrayal. What made the WAAAGH Beast truly dangerous was that it achieved what no other warboss in history has achieved: the unification of much of the Ork race into a single force.
In truth, there wasn't a single Warboss known as the Beast. Rather, the beast was one of six so-called "Prime-orks", each of which were the leaders of one of six ork "legions" that fought during the War of the Beast. It is believed, based on evidence given in the novel series The Beast Arises, that these six legions were the original Ork Klans, as each one had their own flag and position in Ork society that fits with the position of one of the major clans. The more tactically gifted orks, for example, fought under a banner of twin bloodied axes - later the symbol of the Blood Axes klan, famous for using "unorky" tactics such as camouflage and retreating when they are overwhelmed. In contrast, "herders" had a snake, the symbol used by the Snakebites klan - later known for being technologically backwards by Ork standards and relying more on beasts than technology (if so, this would be a fitting alternative, as the herder's job in the Beast's empire was to control the herds of captured humans that were being used as cattle). Judging by both the size of the Beast in comparison to the other five Prime-orks, and his personal symbol, it is possible that he was in command of what would come to be known as the Goffs, a klan known for being larger, more powerful and much more prone to violence than most orks, often producing some of the strongest Ork warbosses.
The Beast is described as many times larger than even the largest warboss, which in Ork society, marks him out as one of immense power, prestige and lethality. He is described as being as tall as a hab-block, with tusks the size of tree trunks. When first encountered by Imperial forces, the Beast was mistaken for a 10-metres tall statue, until it started to move. Another of the Prime Orks was believed to be the size of a small Gargant war machine. This second Prime Ork was armed with two Battle Cannons as well as Autocannons, Missile Launchers, Flamers, and multiple other weapons on a single arm, giving an idea of the kind of weaponry the Beast was armed with.
Aside from his considerable combat and tactical ability, being able to hold his own and even defeat not just primarch Vulkan, but also multiple space marine chapter masters and annihilate the entire Imperial Fists chapter, the Beast seemed lacking in any abilities truly unique for an Ork, aside from the ability to speak Gothic fluently - a language most Orks have only a basic understanding of. The Beast also had very enhanced psychic powers, as it was able to vaporise a Space Marine Chapter Master by projecting some kind of greenish fire with one hand and briefly possessing Space Marines Librarians when they scried the surface of Ullanor from orbit in order to locate him. It could also regenrate his wound via a similar process. Ultimatly, the Beast was so physically powerful that it required Vulkan's sacrifice to (temporally) incapacitate him. However, the Beast's main weakness (and by extension its WAAAGH's) was that it was so saturated with WAAAGH energy that shutting it off could create catastrophic side effects, like blowing the greenskins' heads up in a certain radius. This was exploited by the Imperium to ultimatly kill the Prime-orks, putting an end to their WAAAGH. However, the latent psychic field of the Waaagh that all Orks share was so powerful around the Beast, it caused a rapid progression in culture and society, causing the entire race to evolve - perhaps even to a point closer to their ancient ancestors, the Krork. And this evolution was way faster than they've been since the race devolved into what it was during and after the Great Crusade of the Imperium. This rapid change led to the Orks reaching a point where they were able to create a proper civilisation (at least by ork stardards), similar to the Imperium itself or the Eldar empire before the Fall. This Ork empire was, unlike more contemporary ork "empires" like the Empire of Octarius, a proper society rather than the usual collection of greenskin settlements fighting amongst themselves (until the warboss declares a WAAAGH, that is). Another side effect of this rapid evolution was the WAAAGH having more Weirdboyz than normal, as more Ork psykers appeared amongst the regular population.