Pixies are a form of fairy originally found in the folklore of southwest England, specifically Devon and Cornwall. They are typically more benign than many other fairies, but still mischievous and inclined to cause trouble for humans. In D&D, they were one of four races of fairy-like beings in the original Monster Manual, and seem to be intended as a bit of light-hearted relief, a potentially humorous inconvenience, rather than dangerous monsters to be slain. Of the four originals, they are the only ones to remain in the core monster books for both third and fifth editions.1E
As originally described, pixies look much like elves, except for being only 2'6" (75cm) tall. Or at least, that's what the text says, since the pixie in the picture looks a lot smaller than this. Assuming that the stated figure is accurate, however, it's still a good four inches (10 cm) shorter than a typical two-year-old human child. This makes them the tallest of the four fairy races, and far closer in height to a halfling than, say, a dwarf is to a human. More distinctively, of course, they have two pairs of wings projecting from their back, which look to be similar to those of a dragonfly.
