Gargoyles!

Large stone buildings intended to last for centuries, such as examples of Gothic Architecture, would be exposed to ages and ages of rainwater running down the sides and eroding the mortar. The solution was to devise a channel that would carry water away from the side of the building. The channel was thought of as a "throat" or "gullet" carrying water, and the French term for that was gargouille from the root gar "to swallow."

Not all gargoyles are waterspouts. In Italian architecture, a distinction is made between the carved creature used as a waterspout and called a gargolla or garguglia, and a carving designed simply to carry water away from the building, which is called gronda sporgente, literally "protruding gutter."

A late-14th century legend of St. Romanus explained the origin of the term as the name of a dragon; a good story, but pretty silly.

Long before the Romanus legend, Bernard of Clairvaux in the 12th century commented on gargoyles:

What are these fantastic monsters doing in the cloisters before the eyes of the brothers as they read? What is the meaning of these unclean monkeys, these strange, savage lions and monsters? To what purpose are here placed these creatures, half beast, half man or these spotted tigers? I see several bodies with one head and several heads with one body. Here is a quadruped with a serpent's head; there a fish with a quadruped's head; then again an animal: half horse, half goat... Surely, if we do not blush for such absurdities, we should at least regret what we have spent on them.

The origin of gargoyles that were not waterspouts is up for debate: Bernard doesn't seem to know why they were there. One theory is that they were illustrations of sin or evil, designed to make you glad you were a good Christian. Another thought is that they guarded the church.

If a carving of a creature was not a waterspout, it was a grotesque. The medieval grotesque needs its own explanation, which you will get tomorrow.