The most USELESS monster.

In my opinion, there are no useless creatures among the castle units...

Apparently, that's true; each one can be used for something. As for the neutral creatures, I think the ones that are truly useless are those that I wouldn't take into my army – not even for free, let alone for money. There are a few of those – Zombie, Troll, Peasant, Earth Elemental. Of course, even they can be useful when there's nothing else available...