* The Grim is a creature from folklore, associated with death. It is often depicted as a black dog with glowing red eyes, and is considered a bad omen.
* Sirius Black was an animagus. This means he was a wizard who could transform into an animal at will. His animagus form was a large, black dog.
* The Grim is not related to animagus transformations. While the Grim is often depicted as a black dog, this is a coincidence. Sirius's dog form was simply a result of his animagus abilities.
The confusion may arise from the fact that both Sirius and the Grim are black dogs, and Sirius was associated with death in the series (as he was wrongly imprisoned for murder). However, Sirius was never a Grim.