The Spider Grimm (unofficial) are small arachnid-based Grimm that appear in Vol. 3: From Shadows of the RWBY: Official Manga Anthology, which is mostly non-canon. They are capable of creating physical reflections of people's thoughts.
The Spider Grimm is a tiny Creature of Grimm resembling real-world spiders. They have a black body and three glowing eyes, along with a Grimm pattern on their abdomen. It appears to be able to generate a growling sound.
In Vol. 3: From Shadows, Blake Belladonna is attacked by a Spider Grimm in Beacon Academy, which uses its ability to create reflections of one's thoughts to cause a shadowy version of Blake to attack herself. Once Blake realized the reflection was only a Grimm, she quickly eliminated it.
Weiss Schnee later suggests the Spider Grimm is amongst the Grimm which had attacked during the events of "Breach", having escaped and made its way to Beacon without any of the Huntsmen or authorities noticing.
Powers and Abilities
The Spider Grimm can cause targets to hallucinate a reflection of their thoughts and fears, though it is unknown if this illusions can physically interact with the target. However, they are shown to be capable of "speaking", to Blake's surprise. It is also seemingly capable of producing webs, much like a real spider.