Leprechaun

This small humanoid has pointed ears, green eyes, and a wicked grin. He carries a bottle in one hand and a club in the other.

Source: Pathfinder Bestiary 2

This chaotic neutral fey is the ultimate trickster. Commonly found in forest playing tricks on travelling adventurers. The Leprechaun has a lust for gold that rivals the most greediest glutton. The Leprechaun is not your typical combat encounter. Their main purpose is to harass and play pranks on your adventuring party. 

Leprechauns love gold so much they hoard it away. Maybe in a pot hidden at the base of a rainbow that can only be found under the right circumstances. If a Leprechaun steals an object that is not gold they don't hoard it the same way as gold. They will eventually release that object once the party has become frustrated enough to the Leprechaun delight. Now a stolen bag of gold is another story. The Leprechaun will stash it away to feed its gold lust. 

Rumor has it that if you find a gold coin in the forest and return to a Leprechaun it will grant you a wish. Leprechaun can also be swayed with fine wine and a full stomach. Bringing some vintage wine might entice the Leprechaun to perform a service for you. 

Combat: Leprechauns are small fey with a challenge rating of 2. They have limited combat ability and their spell-like abilities focus on stealth, deception, and stealing. They move around quickly with a 40ft movement which is typically faster then your player characters. 

-Leprechaun Magic: When the Leprechaun uses any of it's spell-like abilities to perform trickery its caster level is 8th level rather then 4th level. They also receive a bonus of +11 to concentration checks. 

-Spell-like abilities: Constant: Shillelagh, At will- dancing lights, ghost sounds, invisibility, mage hand, major image, prestidigitation, ventriloquism. 3/day color spray, fabricate. 1/day major creation.

-Misc Abilities: Damage Reistance 5/Cold Iron, Spell Resistance 13 

Adventure Seeds:

-The Group sets up camp for the night in a forest. In the middle of the night a Leprechaun with invisibility sneaks in and steal an object of value. Maybe something the group is transporting to another town. In the morning clues are scattered about. Following the clues through various obstacles and tricks they eventually meet the Leprechaun who was simply having some fun at the player characters expense. He requires the group to present him with some fine wine before he will agree to part with his newly acquired possession.

-Hired by a local merchant the group is task with finding a Pot of Gold. The Merchant knows that Leprechauns inhabit the nearest forest and hoard their vast wealth in pots. Now the pots can only be found when a rainbow is present. 

-Rumors of a gold coin and wish gets the attention of a nearby farmer. His daughter is dying and his only hope is a Leprechauns Wish. Only problem is the Wish is simply hearsay and group has to discover that the hard way.