A projection is simply a thought psychically projected onto the real world. The ability to create them is far more common than one might think and might account for a vast majority of so-called paranormal phenomena around the world and throughout history, including sightings of cryptids and vampires and even yetis.
Projections can take any form the projector can imagine. They can be monsters, beasts or humans. Or they can be nothing more than shadows. They can even take the forms of inanimate objects. But most projections are, by their nature, extremely fragile. The slightest impact can burst them, usually resulting in a rapid deterioration of the psychic energy comprising it, which may briefly manifest as black dust, ash, smoke or even liquids. A burst projection will leave no trace of its existence behind.
Because of this, most projections are not useful for physical attacks or defense. They are much more effective when used psychologically, either to frighten, distract or otherwise deceive.
In rare cases, a projector of remarkable power can produce projections that are more durable. Theoretically, it might even be possible, with enough power, to create indestructible projections. But even then, they would only be temporary. No projection can last forever, because it requires the projector to remain conscious, and everyone has to sleep sometime.