From Diablo Wiki
Catapults will return as an enemy siege engine in Diablo III. These enemies have not yet been seen in any screenshots or gameplay movies, or even in concept art, but they were announced in an @Diablo tweet in September 2010.
It's presumed that catapults will be found in the frozen northern Barbarian lands, were they were found in Diablo II. Not because they grow there, but because the only known siege in the game is of Bastion's Keep, a Barbarian stronghold players will battle through in Act Three.
Upon the announcement of this type of monster, there was some speculation about what the catapults might throw. 
- What does such a catapult throw? Quite a few things, historically. We often think of catapults or trebuchets hurling huge stones to break down walls or buildings, as seen in the Lord of the Rings films. But they were quite often used for psychological warfare or germ warfare, hurling the severed heads (and other parts) of slain defenders, the diseased, plague-ridden corpses of humans and animals were often hurled towards town wells, and they had other nasty tricks, like baskets full of poisonous snakes or scorpions. There’s a lot of historical precedent for the Diablo III team to use for inspiration.
|Monsters of Diablo III Beasts Beasts: Superunique Demons Demons: Superunique Humans Humans: Superunique Undead Undead: Superunique Objects Bosses Boss Mods Monster Info Fanmade|
Bat - Beast - Blood Hawk - Demonic Hellflyer - Electric Eel - Khazra - Lacuni - Lamprey - Quill Fiend - Rockworm - Sand Monster - Sand Shark - Sand Wasp - Scavenger - Spider - Spiderling - Swarm - Withermoth - Wood Wraith -
|Primary Quest Uniques||
Act One Bosses:
Skeleton King -
The Warden -
Strong CC (Limit 1): Knockback -- Nightmarish -- Vortex -- Jailer
|Act One Uniques||Araneae - Web Spit||Leoric - Summon Skeleton, Teleport Strike, Triple Cleave||The Butcher - Smash, Grappling Hook, Spear, Frenzy Charge|
|Act Two Uniques||Maghda - Forcefield, Summoning Ritual, Punish, Moth Dust, Teleport.||Zoltan Kulle - Fiery Boulder, Time Bubble, Ceiling Collapse, Energy Twister,Vanish, Creepy Laugh||Belial - Phase One: Triple Fireball, Summon Vipers. Phase Two: Poison Meteors, Ground Pound, Lightning Strike, Lightning Breath.|
|Act Three Uniques||Ghom - Acid Slime, Sneeze, Gas Cloud, Chomp.||Siegebreaker - Triple Hit, Charge, Leg Stomp, Grab and Throw.||Cydaea - Pain Bolts, Web Trap, Spider Pits.||Azmodan - Bellyflop, Laser Attack, Falling Corpses, Demon Gate, Pool of Destruction, Globe of Annihilation.|
|Act Four Uniques||Rakanoth - Cleave, Teleport Strike, Blade Strike, Volley, Summon Minion.||Izual - Frost Nova, Frozen, Summon Minions.|| Diablo - Phase One: Claw Rip, Charge, Hell Spikes, Shadow Vanish Grab, Curse of Destruction, Ring of Fire, Ground Stomp, Curse of Anguish, Curse of Hatred. |
Phase Two: Shadow Clone. Phase Three: Overdrive, Lightning Breath.