What is the most useful monster??? Hmm... ladies and gentlemen, it depends on what criteria we use to measure usefulness. There are units that are irreplaceable on the battlefield, there are those that help in managing the castle (peasants), there are those that move quickly across the map, and so on.

In general, peasants are definitely a unique unit (I can't bring myself to call them a monster). It's a cool idea they came up with. They pay tribute (rent) to their lord. This already has a whiff of my favorite medieval times :D