"A drain pipe suffering from intestinal disorder." -Willi Apel
"The instrument gave a fresh zest to Gregorian Plainsong." -Canon Edme Guillaume
Opinions vary about the serpent. Not all of them are complimentary. It's an unlikely looking instrument with a mouthpiece like a brass instrument and side holes like a woodwind. It gets its name from its snakelike shape, not from its ability to play scales.
Here's a video of a Serpent.