Brettanomyces
Brettanomyces is a naturally occurring yeast that can contribute flavors of horse blanket, stable, band-aid, leather, or spice. The yeast can develop at practically any stage in production- in raw materials, barrels, and it is even occasionally intentionally added to the fermenter. It is often viewed as a flaw but there are many styles of beverages in which brett is seen as asset.