Chaos Beasts are a catch-all term for any critters living in the Warp. They differ from deity-aligned daemons as they are born from the raw emotions of the Warp itself rather than a splinter of a Chaos God. They are often native to Daemon Worlds and act as the local fauna and flora. These creatures come in many forms, but each is a creature of muscle, fangs, and claws, with only the thought of carnage driving it. Because GW doesn't see anything unfeasable at all about an ecosystem composed entirely of predators.
Many of these beasts are often captured by Traitor Marines and used either for gladiatorial matches or as glorified zoo animals for all the little Chaos children to look at. It has been rumored that the Great Beasts of Caliban are but another variation of the Chaos Beasts. Although telling the Dark Angels that their Primarch is somewhat related to the Chaos Beasts due to one of his nicknames being, you know, The Last Great Beast of Caliban,
would probably be the quickest way in getting your ass purged other than being the Fallen WHAT!? OUR PRIMARCH EARNED THAT NICKNAME DUE TO HIS HEROIC WRATH AGAINST THE ENEMIES OF MANKIND! THERE ARE NO SUCH THING AS THE GREAT BEASTS NOR THE FALLEN! THEY ARE JUST FAIRY TALES WE TELL TO OUR CHILDREN YES! TO EQUATE HIM TO THE LOWEST CHAOS MONGREL IS HERESY! still doesn't explain why he calls himself a beast if he is a man of honor and valor THE LION IS A NOBLE BEAST! REPENT MOTHERFUCKER!.
Spined Chaos Beast
Right, so you see that picture there? Yeah, let's talk about that thingy that is sadly no longer produced by Forge World. This critter has been around in one form or another in 40K since 4th edition (possibly earlier) - it's been around even longer for fantasy battle, but that's gone now too so...
Anyway, it was first released with downloadable rules as the model preceded any rulebook at the moment. Not much is remembered except that it was a monstrous creature; the first debut was in Imperial Armour Apocalypse in 4th, followed by a mention in the Siege of Vraks during 5th edition initially cast it as a Nurgle-only unit and something of a special character. Those were replaced with a profile from Imperial Armour Apocalypse 2nd Edition to revise things for 5th where it was a heavy support for a chaos daemons army. Though not much is remembered, they were a great alternative to the only other two heavy supports for Chaos Daemons at the time (specifically the auto-take Daemon Princes and the near terminal debut of the Soul Grinder) plus this was before the midstream revisions of flamers or introductions of the seeker and burning chariots. They were okay as far as units went eschewing psychic powers and shooting for better close combat. Their high toughness, strength and weapon skill made them formidable at the task, provided they could survive the shooting that inevitably followed after. Notably, the profile for the Slaaneshi beast was considerably weaker than all the others: whereas beasts of Khorne got more attacks, beasts of Nurgle got FNP and beasts of Tzeentch got better invulnerable saves, beasts of Slaanesh got defensive grenades and it was the most expensive one, too! Fortunately, that book also went out of print and the beast got another update in 7th with Imperial Armour 13 - this time around, the Nurgle beasts got the shaft receiving slow and purposeless and shrouded while Slaaneshi beasts got rending and fleet to replace their lame defensive grenades; meanwhile, Tzeentch beasts kept their better saves while Khorne beasts got free furious charge!
Finally, the age of cheese known as 7th edition ended and now we have a medium-costed tanky monster unit with no model, but some decent rules. It now has separate weapon profiles for its various close combat shenanigans and none of the gods' rules for them is particularly underwhelming - Nurgle beasts got their FNP back, Tzeentchians kept their 4++, Khornates still has Furious Charge technically and now Slaaneshi always strike first! Whether these rules remain is yet to be seen. Warhammer Legends gave it a light update in 9th, making the monster even stronger in close combat by buffing its weapons and special rules.