“I didn’t know leprechaun gold vanishes." -- Ron Weasley (GF28)
The leprechaun is one of the occasional anomalies of the beast/being classification system. Although they alone of the "little people" have the ability to speak human language, they are classified as beasts, and have remained in that category as they have never requested re-classification (although they appear to have made no political statement by this, unlikecentaurs and merpeople. They are able to produce a gold-like substance that vanishes after an hour or two (GF8, GF28). Leprechauns are the Irish National Quidditch Team's mascots (GF8).
In GF, Ron Weasley gives Harry Potter leprechaun gold, thinking he has paid Harry back for buying him the Omnioculars. Ron did not realize that leprechaun gold vanishes and is fake gold.
The term, leprechaun, comes from Irish lupracan, meaning "a very small body."