You needn't call me that. "Madam" will do.
~ Lady Tremaine in Cinderella (2015).
Aristocrats are not powerful enough to be monarchs, but instead are much wealthier, influential and authoritative than your average person. These villains are members of nobilities; Lords, Ladies, Barons, Baronesses, Dukes, Duchesses, Counts, Countesses and so forth. They are frequent antagonists in fantasy settings but exist in others as well.

often they are portrayed as cruel, uncaring or snoobish in regards to any below them in the social hierarchy and depending on the setting they may favor harsh conditions for servants, the use of slaves or the mistreatment of "peasants" - they may also mock those they see as below them and are often seen as very concerned with their own image and wealth, thus often try to appease those higher in status than themselves.

Much like the wicked variety of monarchs most malicious Aristocrats are hedonistic and thus not too concerned with shaping the world to their whim, though some may be much more dangerous than others and may conspire to gain greater power - almost all of them are motived by greed, arrogance and general opposition to the "common man" : as such they are often villains in stories involving folk heroes, such as Robin Hood or other chaotic/neutral good characters.