From "Terrors from above" there's a creature called the Cloud of bones (which can start off as a normal skeleton)
How about a skeleton cobbled together from other types of animals, or perhaps a skeleton which collects bones of its defeated enemies to improve its own power
A skeleton choir/ performing troupe/ circus / orchestra (I had the latter happen to me in a game)
A runescribed skeleton which has the abilities to cast spells based off of the runes on its body
A skeleton which if attacked, falls apart after X damamge, only to get up and attack again, good as new (the magic holding it together either isn't that good or is like that on purpose allowing it to surprize characters)
A skeleton maid/ manservant
A skeleton of X class
...rogue (breaks in and steals things)
...fighter seeks to improve its fighting skills
...cleric wishes to serve his/her god in undeath (or perhaps doesn't realize s/he is undead)
...bard who wishes to sing/ play for adventurers
...barbarian who wishes to get back to his people
...wizard/ sorcerer who casts spells
...monk who finds undeath to be to his advantage
and so on
A skeleton who looks like Skeletor from He Man
A skeleton which insists on acting gentlemanly-- i.e. opens doors for women, pulls out chairs, takes off hat and bows, gets insulted if it's pointed out he's undead and challenges said person to a duel, challenges to a duel those who mistreat/ misspeak of women
A skeleton who has impecible taste in foods and wine... even though it doesnt' have anything to digest its food with.
A Skeleton in a suit of armor
A skeleton bartender who actually knows what he's doing!
A skeleton who murders people so that it can have skin (ala phantom of the opera)
A skeleton which thinks it's a good lover (there's a problem with that... though it may have been in life)
A skeleton of a dangerous monster i.e. orc/ orgre/ beholder (if they actually *have* skeletal structures)