A donkey is a typical depiction of an animal who is stupid, can do whatever it is forced to do. Also, there is a symbol of donkeys in politics, where the Democratic party in the United States is also represented by a donkey (with the Republican Party being represented by an elephant). When people want to protest something, a donkey can be one of the symbols that is typically used; whether that be in terms of taking out a procession of donkeys, or carrying placards on donkeys, or placing an effigy of somebody on top of a donkey. However, in a satirical move, a political party is using a real donkey as a candidate for mayoral elections in Bulgaria (link to article):
The Society for New Bulgaria chose the animal, called Marko, as a candidate for the position in the town of Varna because he "doesn't steal" or "lie".
Angel Dyankov, head of the campaign headquarters of the Society for New Bulgaria party, said: "Unlike the other mayor candidates and politicians, the Donkey has a strong character, doesn't steal, doesn't lie, and gets work done."