Grimm

The Grimm are monsters introduced in RWBY's first episode, which inhabit various parts of Vytal. They are described as "creatures of destruction" and lack a soul; hence they do not have and are unable to use Aura.

History
The Grimm are said to have appeared soon after the dawn of mankind. They targeted early humans, seeking to destroy them and all of their creations. For a while 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 the humans formed their own kingdoms, which grew to survive and prosper.

Although the old kingdoms eventually faded, the world of RWBY still enjoys a time of peace. But this time won't last forever, as in the absence of light, darkness can return and the Grimm will soon make a resurgence. Additionally, the Grimm appear to be the predominant species in the world of Vytal, humanity being limited to four pockets referred to as Kingdoms guarded by Hunters.

Beacon Academy Initiation
The Emerald Forest As part of Beacon Academy's initiation, new students must team up in the Emerald Forest and fight their way to a northern temple. The students soon are faced with several different Grimm monsters. Ruby and Weiss are attacked by a large pack of Beowolves, but are forced to flee when Weiss' attack causes a fire due to Ruby's accidental interruption.

Yang is also attacked, but by two Ursa that only manage to damage her hair, sending her into a blind rage as she defeats one of the Ursa. The other is quickly killed in one strike by Blake. Likewise, Lie Ren is attacked by two King Taijitu, which he defeated through the use of his Aura.

The Emerald Forest, Pt.2 While searching for the abandoned temple, Jaune Arc and Pyrrha Nikos find a cave that they confuse for the temple. While exploring, Jaune accidentally grabs on to the stinger of a Death Stalker, believing it to be one of the relics. Pyrrha then runs out of the cave and the monster follows her out, breaking down some rocks covering the entrance to do so. Once outside, it flings Jaune into the air and Pyrrha flees from the Death Stalker.

Players and Pieces The Death Stalker chases Pyrrha to the temple while Ruby and Weiss are carried there by a Nevermore where they meet up with Yang, Blake, Nora, and Ren. The two Grimm chase the 8 students to a larger part of the temple where Team JNPR are able to kill the Death Stalker by using a plan devised by Jaune: hacking off its stinger and then hammering it into the creature's head.

Meanwhile, Team RWBY were able to defeat the Nevermore by using Gambol Shroud as a giant slingshot and amplifying it using the Dust capabilities of Myrtenaster and the recoil of Crescent Rose's sniper rounds to behead it. The defeat of the particularly fearsome Grimm using the plans of Jaune and Ruby prompted them to be made the leaders of their respective teams.

Within Beacon Academy
The Badge and The Burden, Pt.2 

In the first class of Grimm Studies, Weiss, in an attempt to prove herself superior to Ruby, volunteers to take on a Boarbatusk in a class demonstration. In the battle, the Boarbatusk manages to disarm Weiss but she eventually retrieves Myrtenaster to both block and kill it.

Types of Grimm

 * Beowolves - Wolves
 * Ursa - Bears
 * King Taijitu - Snakes
 * Death Stalker - Scorpions
 * Nevermore - Birds
 * Boarbatusk - Boars

Trivia

 * Grimm wear white masks with red blood trail-like designs, similar to the Hollows from the manga and anime Bleach.
 * The name for the creatures of Grimm may be a reference to the Brothers Grimm or Grimm's Fairy Tales, a famous classic collection of German fairy tales compiled by the brothers.
 * After one of the Ursa is killed by Blake, it began to smoke. This could mean that when Grimm are killed, they will eventually dissolve.
 * The shapes and swirls on Adam's mask are similar to the designs of the Grimm, possibly signifying some kind of connection between himself and the Grimm. It may all be an allusion to Adam representing the Beast of Beauty and the Beast.
 * All of the Grimm creatures vocal sound effects were done by William Orendorff.
 * The Grimm, and the types of them, appear to be either common knowledge or a subject taught in combat schools. This is evidenced by the cadet Hunters being able to identify and name the Ursa and the Death Stalker.
 * As of The Badge and The Burden six species of Grimm were introduced. Such being the Beowolf, Ursa, King Taijitu, Death Stalker, Nevermore and the Boarbatusk. However, in the first episode, during it's introduction, the Boarbatusk was only seen as a silhouette, and was the only Grimm specimen without a proper name. It is later officially introduced in episode 9 as a sketch, and was locked in the cage in Professor Port's class.
 * In Episode 9, every Grimm specimen was seen in sketches in Peter Port's lesson.
 * Due to the apparent lack of a white-and-red design on the head of the smaller Nevermore Ruby crashed into during her fall from the Beacon Cliffs, it can be assumed that Grimm receive these features while or after they reach full size.
 * Or that RT was simply using a preexisting model for a bird, because they didn't have a time to make a new one. So it is possible that the 'Nevermore' Ruby crushes into is just a regular bird, plotwise.