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That's not entirely correct. There are truly useless monsters. They can just stand there and get in the way of other units. (Will a single fairy do anything on the battlefield!?) It takes her 2 turns to fly to the archer. By that time, the enemy units have moved forward and simply killed the fairy along the way. Who did she benefit? No one, she just occupied one stack. The same applies to a large number of fairies (30-40).
Although I also don't agree with Pandora, there are definitely useless units, but I think the point of this topic is to determine the *most* useless monster. So, there's no need to say that all monsters are useful; that's obvious, the topic is about something else.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!