The Great beasts, or Great Siege Beasts, were used by Sauron's armies as burden beasts, most notably during the Battle at Minas Tirith. These beasts pulled the colassal battering ram Grond to the gate of Minas Tirith to destroy the Wall after the smaller battering rams failed, during the Siege of Gondor.
Speculations on OriginsEdit
Great beasts may have indeed been created by Eru himself or perhaps from a long line of giant creatures stemming from Melkor's corruption of the world before the destruction of Almaren long ago during the Years of the Trees.
When in those far back days, Melkor's interference caused the appearance of monstrous beasts of "horn and ivory" that fought with other beasts and were forever marred by Melkor's evil influence, but this was never really explained.
Portrayal in AdaptationsEdit
In film, they are depicted as giant prehistoric rhinoceros-like creatures with an odd horn protruding from their heads. Very little is actually known about these creatures.
Great Beasts are also used as cattle by Mordor orcs in The Lord of the Rings: The Battle for Middle-earth, and in The Battle for Middle-earth II very similar looking but fatter animals are used also by dwarves as burden beasts and are referred to as yurgs or subterranean yaks.