Beastmen
Where the Warriors are Chaos tanks and Daemons are Chaos cheese, the Beastmen are the true Chaos horde. The Beastmen call themselves the children of Chaos, descended from outcast mutants hurled into the cursed woods of the Drakwald. They are considered a mere nuisance by the Empire and the Bretonnians, but the Wood Elves know how dangerous they truly are. The favorite activities of most Beastmen include toppling waystones, erecting herdstones, kidnapping/raping/eating humans, and, of course, plain old smashing shit! They hate absolutely anything built or anything that is pure and make it a priority to destroy and defile that which the civilized races cherish. In short, they're a bunch of drunken anarchist fucktards that hang out in the woods, surfacing only to pillage and defile - which is actually kinda awesome if you're into that sort of thing!
Print History
The Beastmen weren't always "the Beastmen" as we know them today. For the longest time, they were actually part of a unified chaos army, but due to the fact that people seemed to like to roll with either all beast, mortal or daemon army lists, Games Workshop in their infinite wisdom decided to gradually split off the armies. Despite getting splitoff into an army of their own, they still always play to fiddle to, every other villain faction in the setting.
Beasts in Hordes of Chaos
Squishier, quicker units than mortal warriors and good for setting up charges, which daemons couldn't really do. A couple of special and rare units that were "beastly," but nothing much.
Beasts of Chaos
They were pretty bad in this edition being essentially the bitch of chaos and with almost NO redeeming qualities of their own. When they were first split from the Hordes of Chaos some years ago, they were nothing more than an unruly (literally) band of meat shields for your warriors and daemons. In fact, because they could move faster, it was preferable to field a few beast packs to soak up arrow fire and act as distractions while fast-moving daemons got the flanks and warriors hammered the front ranks.
They had core units that mixed low toughness, lightly armoured Ungors with spears and tough, brawny brawlers in the form of Gors. They were also skirmishers that could essentially scout and would lineup with the Gors in the front and the Ungors jabbing from behind with their spears. The alternative was to supplement with warhounds or take a bunch of strong, slow bestigors, but they were too expensive and almost entirely useless.
The only interesting thing about the Beasts was that they had access to a lot of cool special and rare choices, like Chaos trolls and Ogres, Dragon Ogres and Shaggoths, etc. However, Hordes of Chaos had better shit and later on, the Warriors of Chaos would absorb almost all of the Beasts' specials and rares!
Beastmen
Sometimes referred to as "the WEAK chaos army," it more or less resembles a normal horde style army in Warhammer Fantasy Battle with cheap, expendable troops, average leaderships and little immunity to psychology. The things they otherwise don't have are anything resembling armor or any kind relevant shooting, but they make up for it with Primal Fury which gives them hatred on a passed leadership test each round of combat on many of their units. The Herdstone magic item allows Beastmen wizards to score extra power dice for their spells, allowing Beastmen to pull off a lot of spell support to augment the cheap and reliable core units. They also excel in cheap chaff and redirectors in the form of Ungor Raiders, Razorgors, and Harpies, and fielding fairly costed chariots in core. The biggest change was the totally unnecessary nerf to Ambush - which now only works on a fucking 6, making it basically unusable.
The Beastmen today are fair, if weak as a stand-alone army featuring decent core choices that can win through the support of augmentation and hex spells, and colorful selections of heavy hitters like the (overpriced) Ghorgon and the (totally underused) Doombull.
Occupy Drakwald
The thing that pisses the Beastmen off is that something like 1% of life in Middenheim controls 99% of the land! There are so many freaky beasts living in the chaos woods, combined with big empire pushing around the people of the woods and ruining the environment for their own economic interests. And so the Beastmen have taken to their eternal campout against the empire and it's regional castellan pretty boy, elector count Boris Todbringer.
And that's where the similarities end. The Beastmen are a bunch of lazy, shiftless, filthy degenerates that eat babies - no literally! That wasn't an anti-progressive political jab at all, it's the plain truth. The beastmen want nothing better than to just drink looted booze, eat/fuck other races and tear down everything that everyone has ever built - ever!
Origins
The Beastmen's origins seem to be something of a composite piece. On one account, they are descended from the myriad creatures that inhabited the Northern Wastes and migrated downward, accepting more stable forms from their reduced exposure from the polar fluxes of chaos and interbreeding with more magic resistant creatures. The other part of the myth has pins the Beastmen's origins as the mutated descendents of humans cast into the wild for being born a hideous half-man, half-beast creature. Either leads to the Beastmen of today being eternally butt-hurt over being left to die in the Drakwald. Now why the other races (notably the humans) decide to throw all their mutant babies into one forest known to be populated with sadistic and vengeful goatmen born as mutants to humans and cast-off themselves is a bit of a mystery. Wood Elves seem to have less of a problem with mutant kin, either because they resist chaos more/better or they kill such offspring immediately and never talk about it again, but that's just like, you know, a cultural difference.
Regardless, today's Beastman is more likely either the product of gangrape, spawned by a brief and unholy union from victorious Beastman raiders that managed to bring home some captive females from other races (usually humans, and it never ends well for the woman), or they were spawned when a Beastman really loves a Beastwoman. We're not too sure how Beastmen "love" one another, but the fluff seems to imply that the Beastmen have to get really fucking hammered before hand with giant orgiastic feasts at their encampments around Beastmen Herdstones.
Herd Society
Beastmen society is organized around herds and ruled by a system of might makes right. In other words, what the biggest, strongest Beastman (or Beastwoman) says, goes. One would be forgiven in thinking that this automatically means that minotaurs would end up running the show since they are by far the biggest and strongest, but they're not the brightest thing on two hooves. Minotaurs specialize in wholesale slaughter, but aren't too keen on things like where to find food, who should keep watch or how to fix their shitty chariots. Lo, the Beastlord has his task cut out for him.
- Beastlords: the Beastmen in charge, they run the show for the other Beastmen with a combination of dastardly cunning and brute strength. They often delegate more benign tasks to their immediate inferiors, the Wargors.
- Wargors: the "trusted" (as far as Beastman loyalty goes) lieutenants of Beastlord, Wargors are pretty kickass leaders and fighters themselves. Oft seen carrying battle standards or leading Bestigor herds, or both.
- Bestigors: quite literally the best Gors, a ha ha </sarcastic laugh>. Strong, tough, heavily armed and armoured, the Wargors and Beastlords often arise from their ranks.
- Bray and Great Bray Shaman: tribal elders and spiritual leaders of the Beastmen, they interpret the will of Chaos and determine the best course of action for the heard. They are also the herd's wizards.
- Gors: the rank and file of the beast herd. Your average Gor is bigger, tougher and far nastier than your average human. He's also an unguloid with furry legs and a face that's a blend of bestial and human features. ALL Gors also feature prominent horns and/or antlers on their heads.
- Ungors: the untouchables even in beast herds. They are pretty close in size to humans, yet still feature particularly bestial features such as furry unguloid legs with hooves, a semi-bestial face and two small horn buds. Sometimes band together in scouting parties of Ungor Raiders when not used as slave labour to fix all the shitty Beastmen gear.
- Breyherd: Beastmen with no horns, the lowest of the low in Beastman society. In a society where life is short, brutish and nasty, their lives are by far the nastiest, most brutal and most especially shortest.
The Bulls
Nobody really knows why, but the Beastmen seem to be accompanied by a race of ferocious minotaurs, not that they're complaining. When the minotaurs aren't rampaging through the hardest parts of an enemy army, eating knights, drinking blood and shitting victory, they're often seen guarding caches of magic treasures deep in the Drakwald. They aren't a very bright group but their uncanny resilience, rape-tastic strength and general piss-your-pants scariness gets shit done for the Beastmen. There are also mutant varieties of the bulls that have long ago traded their sanity for fuck-off hugeness and other random shit.
- Doombulls and Gorebulls: The closest thing the minotaurs have to leaders, with the Doombulls being the larger, stronger and generally more daunting of the two.
- Minotaurs: Giant, bull-headed monsters that fuck near can't be stopped once they get to smashing shit.
- Cygors: 15 ft. tall, one-eyed minotaur mutant that can only see magic things. Likes to collect gigantic pieces of magic rock (more commonly known as temple columns) and eating the souls of mages... unless those tricksy wee things are causing him trouble, whereupon he'll just throw one of his gigantic rocks at it!
- Ghorgon: minotaurs that get a flair for cannibalism turn into giant-sized, four-armed killing machines with two of their four arms ending in scythe hands. They also have a penchant swallowing things whole and magically reknitting any wounds they may have had.
Other Critters
Being that the Beastmen are all a bunch of horrible monsters, they have a pretty high tolerance for other horrible monsters. Along with the minotaurs, the Beastmen's ranks are supplemented by other creatures used for various tasks from beasts of burden to cavalry to shock troops to cannon fodder.
- Warhounds: just like the ones starved and taunted by the warriors of chaos, but instead starved and taunted by Beastmen. Used for a fast and cheap screen
- Tuskgors: what essentially look like warthogs, they're most often used to pull the herd's shitty chariots.
- Razorgors: bigger, nastier, mutant tuskgors. They are either loosed in packs or used to pull chariots. Why the Beastmen use pigs as pack animals and eat horses instead of the other way around is a bit bizarre, but might have to do with how flighty horses can get around large, carniverous animals.
- Centigors: The torso of a Gor fused onto the body of a horse with three hooves per foot, by the looks of it. Totally ungainly and butt-hurt over it, they take to stealing beer, wine and spirits brewed and distilled by other races and getting shit-faced daily to forget that fact. They act as cavalry to the Beastmen.
- Harpies: identical to Dark Elf harpies, may or may not be the Beastwomen with which the Beastmen procreate. Used primarily as flying shock troops.
- Chaos Spawn: not sure if either heavily mutated Beastmen or just normal spawn, that which must not be named are goaded into battle... somehow.
- Jabberslythe: arguably the ugliest thing in all of WHFB. Said that not even the clearest pools will reflect its image, or maybe they just don't stay clear for very long. Either way, this is yet another giant powerhouse of a monster using its ugliness as a shock weapon to get the Beastmen's enemies to kill themselves.
- Giant: the warhammer giant is at the beast's disposal to jump up and down, yell at people and stuff them into its pants.