The most useful monster??? Hmm... 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), and there are those that move quickly across the map, etc.

In general, peasants are definitely a unique unit (I can't bring myself to call them a monster). It's a cool concept. They pay tribute (rent) to their lord. This already has a medieval feel to it :D