Many of the monsters in what Dave "Sham" Bowman called the "Fairy Tale Miscellany" are under-represented in most D&D. Of course they aren't exactly a typical combat encounter, but more and more I've been of the opinion that "weird" encounters can and should be a staple item of old school gaming. Caverns of Temeluc in Dungeon Crawl #2 features a whole counter-party of gnomes, and Demon Haunted Lands will deal extensively with faeries and their realms. So, about those pixies. Or piskies, or however you care to spell it.
When pixies steal horses, as they are wont to do, they ride them in circles and form what are called "fairy rings" or gallitrap, made of dirt or ringed by mushrooms. These are a game-ready concept: a person who sets one foot inside a gallitrap can see pixies, but if they set two feet in they are trapped, and falls to their power. Criminals who set foot in a gallitrap will be hanged. Such an encounter is a quick and classic oddity for wilderness exploration.
Generally pixies harass travelers, and are either invisible or so small they cannot be seen. In the case of particularly boorish types, they will mislead them into a river or a bog - this should be handled by illusion, or by false trail markings, adulterated signs, etc. Characters who are carrying a cross/holy symbol or bread may be able to ward off the pixies, or to buy their cooperation, but disrespect is not tolerated. Gifts of clothing may also flatter the pixies and win their non-interference.
Horses - particularly ponies - may have an ability to see otherwise invisible pixies. This is frequently used to their riders' detriment, as the pixies may use this and the animal's tendency to avoid them to herd their riders to their (unwanted) destinations.
I think there is always a danger in RPGs of making too much of "prankster" characters and having them become annoying; pixies instead should be territorial but not homicidal. Hints of where they live should be available from farmers and old wives, and "Piskie Country" should be land that is only entered by seasoned adventurers with extreme respect for its inhabitants.
This idea of "not always combat" encounters in the OD&D bestiary is one I'd like to spend a bit of time with, as I think it's a widely under-appreciated