"You should be in class, Sonny Jim."
"This is urgent..."'
"Ooooh, urgent, is it? ... Well, that's put us in our place, hasn't it?"
-- Staff Room Gargoyles to Harry (OP17)
Gargoyles are a semi-sentient stone creatures used as guards for rooms in Hogwarts.
- A stone gargoyle blocks the entrance to the Head's Office. It steps aside when the correct password is given to enter (CS11).
- A stone gargoyle is used as a water spout in the Potions dungeon (PA17).
- Two stone gargoyles guard the staff room and sarcastically challenge students who attempt to get into the room, once referring to Harry as "Sonny Jim" (OP17).
From Old French gargole, gargoule "throat," from the Latin "gargola" - the root of the word "gargle" in English. "Gargoyle" refers to intricately carved stone downspouts (throats) for water drainage on buildings, often in the shapes of hideous or whimsical monsters.
Many are found on old Catholic cathedrals such as Notre Dame in Paris or on university buildings around the world.
The term Gargoyle is also used as an interjection ("Gulpin' Gargoyles!") and as a disparaging nickname for someone who is mean and unbending (e.g. "Yeh don' know them gargoyles at the Committee fer the Disposal o' Dangerous Creatures!"), rather stupid ("And if you'll believe me, lads, the gormless gargoyle buys all 'is own toads back orf me for a lot more'n what 'e paid in the first place -"), or even completely despicable ("That foul, lying, twisting old gargoyle!", which Hemione said of Umbridge).
Slytherin student Greg Goyle's name seems to be a play on the word Gargoyle, if you say it quickly.
From the Web
"Gargoyle" on Harry Potter Wiki
"Gallery of Ghastly Gargoyles Around the Globe" from The Travel Channel