A pixie is a one of several nymph-based monsters introduced in SLASH'EM. While it lacks the seduction attack of a nymph, it is slightly faster, has a significantly lower armor class, and is always generated invisible.
Like nymphs in vanilla NetHack, pixies are always generated asleep. Unfortunately, this is not so useful, as it is quite easy to blunder into them if you can't see them. Once they are awake, they are a major threat, as they are quite likely to steal an item and teleport away before you can respond. Fortunately, they are only able to steal one item at a time. To fight them, you need a means to see them. The best way is to have see invisible, either through the ring, potion, or intrinsically. Failing that, a somewhat easier option for a low-level character is to have telepathy and a blindfold or towel. Once you have a means of detecting them, they are no more threat than a usual nymph; their lack of a seduction attack more than makes up for their slightly increased speed and better AC as compared to normal nymphs.
Do fairies live in the hedge by the lawn?
With pale, violet eyes and wings that shimmer?
Not in my garden of thistle and thorn,
Well, not since I tidied it up with my strimmer.