A barnyard, horse blanket, or sweat-soaked leather aroma in a wine may be a sign of:

D.

Brettanomyces (aka Brett) is a type of yeast that causes aromas and flavors politely known as “barnyard.” Brettanomyces can exist in cellars, barrels, and hoses, and from there, migrate into wine. Because it is a yeast, Brettanomyces can grow in the wine, causing a more severe barnyard character with time. Critics of brettanomyces say it robs a wine of its true flavors and has no place in winemaking. But supporters find that small amounts add a certain complexity  to wine, and can give it an aged flavor.