Leprechaun

The Leprechaun, or Clauricorn, (Homunculus Calceator) is a a species of sapient Little People.

Physical Appearance
Leprechauns are six inches to one foot tall. Their skin can be anything between human-like tones to a green color, much like goblins. They have small noses, a wide mouth and large pointy ears.

Diet
Leprechauns are strictly herbivorous, their diet consisting mainly of leaves. Though they do not brew it themselves, they also enthusiastically consume alcohol if they can purchase it from other species.

Civilization and Lifestyle
Leprechauns only live in Ireland. They live in small villages in wooded areas, and have a rather human-like society and culture. Leprechauns, while they lack the clout to fight it actively, are opposed to the Statute of Secrecy (much like the also-Irish Banshees), and often make contact with lone muggles (hence why they are so prominent in muggle folklore). While lastingly revealing magic to the muggles could get them in trouble, leprechauns take advantage of their classification as Beasts rather than Beings (which therefore does not bind them to obey wizarding laws) to avoid sanction over these small offenses. In the recent past, leprechauns often sold objects of their making (such as shoes) to the local wizards and muggles, though this time of trade has greatly lessened today.

Magical Abilities
Leprechauns are among the species covered by the Wand Ban, forcing them to rely on wandless magic instead of spells. They have developped a unique wandless spell to conjure large amounts of gold out of thin air; unlike typical conjuration spells, this gold is magically indistinguishable from true gold, but it will vanish after a few hours. All attempts by wizards to replicate this spell have failed, but it has become famous enough that it is useless to Leprechauns as far as money is concerned, forcing them to use it only for pranks.

British Ministry of Magic Classification
Leprechauns were classified as Beasts by the Ministry (which, while obviously unfair, is actually to their advantage as seen above) and given the threat level XXX.

Canon

 * Leprechaun on the Harry Potter Wiki