Great Beasts

The Great Beasts were chimeric creatures native to the planet Caliban in Games Workshop's Warhammer 40,000 universe. These beasts were the most dangerous creatures on the planet, believed to have been created due to the Caliban System's proximity to the Eye of Terror. The species was driven extinct by the primarch Lion El'Jonson during his time as Supreme Grand Master of the knights of The Order, after he declared war on the Beasts. Even if one of these creatures managed to survive the war and the coming of the Emperor, they would've been wiped out at the end of the Horus Heresy, after warp storms tore the planet apart during the civil war between the Lion's loyalists and the traitorous followers of Luther.

Appearance
All Great Beasts were chimeras, though the exact form differed between the individual Beasts. Despite this, only one kind of beast could truly be described as unique. One of these creatures, encountered by the Order knight-supplicant Zahariel in the novel Descent of Angels, is described as "something dark and winged [...], it's body scaled and reptilian," with "claws like swords," a spined head on the end of a serpent-like neck, filled with "razored fangs like the teeth of a woodsman's saw," and translucent, filmy wings, edged in ridges of horny carapace and ending in long, barbed claws. Another, encountered by Zahariel and his cousin, Nemiel in the fortress of the Knights of Lupus after they'd become full knights, was described as a bear-like creature covered in spines, whilst a third was a lizard-like creature with fangs so large, they stopped it's jaw from closing.

As stated above, whilst the Great Beasts were in no way unique from each other, there was one exception - the Calibanite Lion. A rare kind of Beast, only two of them were ever discovered, each killed by Lion El'Jonson and Zahariel respectively. Again, from Descent of Angel, a description is provided for the one Zahariel fought, detailing a wide and powerful creature that was "leonine only in that it was a quadraped with a mane of blade-like spines growing from behind its head." Each of it's limbs was covered in plates of natural armour that was described as having "the quality of rock, yet the pliability of flesh," which could patch itself and shift to where it was most needed. Knife-like claws extended from its front paws, and a pair of long, lethally sharp fangs protruding from it's upper jaw. Finally, they have a pair of orange eyes with reptile-like, slitted black pupils.