Creatures of Grimm

The Creatures of Grimm, or also simply known as Grimm, are creatures that inhabit various parts of Remnant. They are described as "creatures of destruction" and lack a soul; hence, they are unable to use Aura. They are in turn also drawn to feelings of negativity such as envy, sadness, loneliness, and hatred and will often congregate towards the source of these emotions.

At one point, ancient cultures believed that Grimm were animals possessed by evil spirits, or were the spirits of once tortured animals.

RWBY
The Grimm were created by the God of Darkness in order to destroy his brother's creations. Grimm have targeted early humans for as long as can be remembered, seeking to destroy them and all of their creations. They served as enemies and beasts throughout the world, making people fear them and the God of Darkness. However, due to a human rebellion against the gods pioneered by Salem, the world was wiped clean of all life except Salem, leaving the Grimm and her behind. Salem, cursed with immortality from the God of Light's Fountain of Life, tried to kill herself by diving into the God of Darkness' Grimm Pool, where all Grimm came from. The pool did not kill her, but rather transform her into the monster she was and gave her authority over the Grimm, who sought to continue their mission to destroy life, especially humanity.

At first, it seemed as though they would succeed as humans did not have the strength to fight them. However, humans discovered the power of Dust and with it, the Grimm were driven back. During this time, humans enjoyed a time of peace and soon formed their own kingdoms, which grew to survive and prosper. However, this time will not last forever.

While Grimm are initially mindless and heedless of risk during their youths, to where they will simply attack any humans on sight, some Grimm are so powerful that they have survived to live for hundreds of years, and in that time they have evolved and learned from their experiences fighting humanity.

In doing so, they have exhibited restraint and the patience to avoid taking unnecessary causalities in futile conflicts. Instead, they stay close to humanity's borders, waiting for the opportunity to come where they can finally strike, and thus mankind is always in ever present danger, even in times of apparent peace.

Mega Man ZX Shippuden
In the Fan-made series of Mega Man ZX Shippuden, the Grimm were created by Suzugamori Ōtsutsuki in order to destroy his family's creations. They served as enemies and beasts throughout the history of Dens, an ancient planet of the Dynapaul System. However, due to a human rebellion against the gods pioneered by both Salem and Satan Ōtsutsuki, the world was wiped clean of all life except Salem, leaving the Grimm and her behind. Salem, cursed with immortality from Yahweh Ōtsutsuki's Fountain of Life, tried to kill herself by diving into the God of Darkness' Grimm Pool, where all Grimm came from. The pool did not kill her, but rather transform her into the monster she was and gave her authority over the Grimm, who sought to continue their mission to destroy life, especially humanity.

At first, it seemed as though they would succeed as humans did not have the strength to fight them. However, humans discovered the power of Dust and with it, the Grimm were driven back. During this time, humans enjoyed a time of peace and soon formed their own kingdoms, which grew to survive and prosper. However, this time will not last forever. In doing so, they have exhibited restraint and the patience to avoid taking unnecessary causalities in futile conflicts. Instead, they stay close to humanity's borders, waiting for the opportunity to come where they can finally strike, and thus mankind is always in ever present danger, even in times of apparent peace.

Biology
Further study has disproved this theory over time, however, due in part to the discovery of even more horrific and misshapen species of Grimm which have no animal counterparts. With the discovery of new types of Grimm every day, scientists are left with more questions then answers.

Grimm come in a variety of shapes and sizes; the latter appears to be a factor of age, and are said to be the only creatures without souls, thus being deprived of the use of Aura, but making up for this with strength and savagery. Grimm are stated to be attracted to generally negative feelings such as sadness, hostility, anger, fear, and many more, and even congregate to areas that, although humanity has long since abandoned, still hold residual traces of these feelings.

This behavior will even lead to them to join in on an attack in progress if the humans being attacked begin to panic. Grimm typically form packs or other types of large groupings with other members of their own specific species. While some lone Grimm may stray from the pack for hours or even months, they will inevitably rejoin their group to continue their instinctive drive to hunt the people and destroy any artificial creations associated with them.

The longer a Grimm lives, the larger it becomes. This added size comes with an ability to learn from their encounters in contrast to the Grimm's more reckless nature during their youth. While sometimes requiring hundreds of years, the Grimm's accumulated experiences over the course of surviving their battles with man can cause them to begin exercising caution.

This perverse form of self-preservation can even lead them to avoiding unnecessary conflicts altogether. Yet, despite this ability to learn, their instinctual hostility still remains, as shown by their preference to patrol the borders of Kingdoms for any weaknesses they might someday exploit. This demonstrates that for all of their apparent intelligence, older Grimm simply use it as a means to become more effective in their singular drive and purpose, to kill.