BX Monsters A to Z: Gargoyle

 In medieval times, they were architectural flourishes, placed on pillars and rooftops as silent guardians against evil spirits. In D&D, they tend to be evil themselves, or at least Chaotic and hostile. They're gargoyles, from the Basic Set.

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Gargoyles are immediately stated to be magical monsters which save as Fighter:8. While that's a significant bit of information, it's far from the most interesting thing about them and the saving throw is readily evident in the stat block. It really seems as if Tom Moldvay struggled a lot in coming up with opening sentences for monster descriptions. Anyway... Gargoyles can be harmed only by magic or magical weapons, which right away makes them a serious threat to Basic-level characters. Like their real-world inspiration, they are described as horned, clawed, fanged, winged, and hideous, with skin that often looks exactly like stone, so they are sometimes mistaken for statues. The word often is an interesting qualifier, implying that sometimes they don't look like stone. No other examples are given, so it's left up to the DM if some gargoyles in the campaign might appear to be wood, metal, flesh, ice, crystal, or some other material. The word looks is also significant, as it indicates that a gargoyle is not necessarily actually made of stone, but only has the appearance of it. 

Their AC of 5 seems to bear out this hypothesis, reflecting a tough hide but not as hard as stone... not that they need it when facing opponents who lack the magical means to fight them. Their 4 Hit Dice enable them to withstand a fair bit of punishment, even if the party has magical weapons. None of their attacks alone is terribly impressive (two claws for 1-3 each, a bite for 1-6, and a horn attack for 1-4) but four attacks per round is pretty likely to yield at least one or two successful strikes, and against lightly armored targets, you could be looking at as much as 16 points total. Their ground movement of 90'(30') is adequate to keep pace with most adventuring parties, and a flying rate of 150'(50') gives them a solid advantage if they can take to the air. With a Morale score of 11, they're highly unlikely to give up a fight or a chase, so the PCs had better be clever and lucky if they want to escape an angry gaggle of gargoyles. 

Gargoyles are stated to be cunning and at least semi-intelligent, and they also attack almost anything that approaches them. Since they're rarely found alone (1-6 appearing in the dungeon; 2-8 in the wilderness) it seems likely they could utilize some sort of pack tactics, making them still more dangerous. They are immune to sleep and charm spells, so Basic-level parties can't rely on two of their biggest trump cards for opponents they can't best in straight-up combat. The final sentence of the description advises DMs not to use gargoyles unless the party has at least one magical weapon, but honestly, one or two magic weapons is not likely to change the ultimate outcome of a battle between a party of level 1-3 and even a single gargoyle.

The concept of "constructs," and the inclusion of gargoyles in that category along with golems, living statues, and other monsters magically created and animated from stone and other materials, did not come into the Basic D&D game until the Mentzer Companion Set, so far as I can find. That means the exact nature of gargoyles and their origins are left open to the DM's whim. I've always imagined them as things with both magical and biological aspects, hatching from eggs and living in family groups. If you want them to only be products of high-level spellcasters animating inanimate material, though, or beings from the elemental plane of earth, or faeries or demons or what-have-you, B/X canon says nothing to contradict your preference.