Monsters are an interesting subject in my story Eye in the Sky. The fact that most people can't even perceive them, and those who do are more or less are treated as a food source for monsters. Make them hard to be studied without a major risk of injury or death.

Monsters are grouped into 3 categories. Mutated, created, and Mana born. Despite the monsters in each category being greatly different from each other in origin, they all share 3 things in common. 

1. All monsters need Mana to survive one way or another. Some need it for something simple, like allowing the monster to support its weight, while others might need Mana to reproduce, some even need Mana to breathe. The way monsters use Mana differs from one species to another. All monsters use Mana one way or another to live. Like how a lion will eat a zebra to survive, a monster will eat a Mana filled organism to live.  However, because they only eat organisms with Mana or those that produce Mana, they will completely ignore any organisms that cannot contain/produce Mana.
   

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2. All monsters can't be perceived by those who don't contain/ produce Mana. In contrast, monsters can see and interact with organisms who don't contain/produce Mana. Which makes those organisms completely defenceless to the monsters. However, you will never see a monster intentionally harm a Manaless organism as they have no reason to do so. Any harm they do to Manaless organism are either by accident or because the Manaless organism got in its way. This however doesn't make them empathetic creatures. They simply don't see a reason to actively harm Manaless organisms, like how a lion sees no need to actively hunt a tree.
   

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3. All monsters contain a core. What's a core? Well, a core is the concentration of Mana in a monster. A core is a monster's way to store Mana for later use. Without a core, a monster would be in a constant state of Mana "starvation". Imagine being constantly hungry because your body can't store any energy or nutrients you would have gotten after a meal. This can cause a monster to enter a state similar to malnutrition, and will eventually lead to the monster's death. However, this is rare as removing a monster's core would likely kill the monster long before it can "starve". Removing a monster core, and letting live is seen as a very cruel act to many in the Hidden World, it's like cutting a monkey's hands-off and releasing it back into the wild. It's also a very risky thing to do because if a monster survives long enough they can regrow a core. When this happens they would be are considered Berserkers.  This will always cause the monster to be stronger, more unstable, many times more dangerous than it was before.  When this happens the casualties that come from that monster's rampage can easily reach the thousands. To Manaless people this rampage would be perceived as a natural disaster equivalent to a hurricane. That's why it's illegal to take a monster's core and not kill it

The 3 monster categories.

• Mutated- For a monster to be considered a mutant they have to be Manaless in origin.  Meaning they either were born Manaless or have Manaless parents. The monster also has to start to produce Mana because of a natural occurrence, meaning a monster who started producing Mana due to human intervention isn't considered a mutant, they would be considered created. Many monsters that are created this way don't tend to leave any offsprings. However, if they do manage to create offsprings that can reproduce, they would be considered the first generation of a new Mana born monster species. It's currently unknown how mutants are created, similar to how people don't know how people become awakened.  Mutated monsters look very similar to their base form. For example, a mutated boar monster would look very similar to a normal boar. The change that the Mana causes can be subtle like being slightly bigger, or it can be something major like being able to breathe fire. A mutant monster's core can always be found either in the heart or in the brain.                                                                                                                             
  

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• Created- For a monster to be considered created they have to be Manaless in origin and forced to produce Mana through unnatural means. A good example of a created monster would be a golem, a pile of rocks brought to life through human experimentation. A created monster does not have to be made by a human. Some monsters can raise the dead, those raised creatures would be considered created monsters, despite their creator not being human or a created monster themselves. If left to random chance the changes that Mana causes to a created monster are very similar to that of a mutant monster. However, a created can be moulded to its creator's liking. A good example would be a chimera whose body can be vastly different depending on how the creator made it. A created monster's ability to reproduce is the same as that of a mutant monster. It's mostly up to change if they can produce offerings or not. And if they do manage to create offsprings that can reproduce, they would be considered the first generation of a new Mana born monster species. A created monster's core can be anywhere the creator pleases, but it's usually located in the heart or the brain.
  

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• Mana Born- For a monster to be Mana born they have to be able to produce Mana the moment they are born and have parents that can produce Mana themselves. The first generation of many Mana born monsters usually isn't Mana born themselves. Mana born monsters are stronger than the other two types of monsters and thus need more Mana to survive. This means they either produce more Mana, or hunt other Mana filled organism more often. Their core can be found around the area where they would require the most Mana, which usually is the heart. Mana born Berserkers are the most deadly types of Berserkers, while also being the rarest. As Mana born monsters are more reliant on Mana to survive than the other types of monsters.