The Zamorakian siege beast is an extremely large and powerful behemoth serving Zamorak, that appeared in the final week of the Battle of Lumbridge. It would have appeared earlier on in the battle depending on the outcome of the second Zamorakian vote. It is the only known behemoth found outside of Daemonheim, because Zamorak took it with him during his escape. The second vote resulted in favour of the Zamorakian champion. The eighth vote again gave the behemoth the possibility of appearing in battle, but resulted in favour of the Zamorakian butcher demon, meaning it still didn't make it into the game.
On the final week of the battle, they were finally released, and it can be found standing by the entrance to Zamorak's camp.
They fire huge flaming rocks at Saradomin's units, which deal a large chunk of damage to them. The Siege beasts are capable of dealing up to 32,000 damage on their Saradominist counterparts, the colossi.
Like all special units, they do not retaliate against players unless they are not currently under attack or are attacking any other target.
Killing it gives six times their current multiplier in divine tears. Merely participating in the kill is enough to yield the tear reward - it is not required to deal the most damage.
The Zamorakian army summons a fearsome behemoth. (link)