#because it's restricted to Small or larger projectiles and cannonballs are

[monstersdownthepath]

Rock catching is such a baffling ability for monsters to have, especially as a Universal Monster Rule. Were designers really expecting players to be hurling boulders at enemies regularly enough that giants catching them would be a "gotcha"? Or did they expect DMs to regularly simulate giant-on-giant violence? And are they any creatures that have rock catching but not rock throwing, or vice-versa?

I do agree Rock Catching and Rock Throwing could have been rolled into a single ability, but ironically, yes, there are creatures with Rock Catching that don't have Rock Throwing, and vice-versa;

Throws but can't Catch: A lot more than you might think; too many to list, but I'll just rattle off the highlights I find funny: Sturzstromer (which are made of rocks), Cherufe (which are both enormous and made of magma), various Cyclopses, and the Hekatonkeires for whatever reason.

Catches, but can't Throw: Jungle, Sun, Storm, and Wood Giants, along with the Plankta and Ulkreth by technicality (they can, through abilities, throw a lot of rocks, but they do not have Rock Throwing)

Anyway, the reason Rock Catching exists? There's a few I can think of off the top of my head: Namely, the first is an in-universe explanation: it makes sense they would learn to defend against the most common weapon others of their kind would use against them. Secondly, it makes sense they would learn to defend against the most common weapon that Small Things can actually realistically use to harm them.

I speak, of course, of the catapult.

Rock Catching doesn't specify how the flying rock must have become airborne to be able to snatch them out of the air. Whether it's because of an avalanche or cave-in, an enemy's Rock Throwing attack, or a projectile shot out of a siege engine, a creature with Rock Catching can stop them short with a relatively low Reflex save and throw them back next round. Because the ability so rarely ever comes up against players, a party attempting to bring an army against a well-fortified enemy can enjoy a nasty surprise as they have their ranged siege weapons completely neutralized if the BBEG brings in even a small handful of Giants.

And guess what? Rock Catching is even more versatile than it looks! Much like how Rock Throwing specifies that a 'rock' is any object of a specific size and made of a relatively uniform material, allowing a Rock Thrower to hurl nearly anything as an improvised weapon, Rock Catching says it works on rocks or similar projectiles. Hope you're not firing casks of burning pitch or other sorts of exotic ammunition, or you may be eating your own attacks the very next round.

How's that for a 'gotcha?'