Entities come in all numbers of dimensions. It has nothing to do with if they are 'positive', 'negative', 'good', 'evil', or 'bad'.

The entities referred to most often in Robert Bruce's literature and in Toltec literature (allies) are 'low level' entities from the lower astral planes. They are the most common type and they can easily access the Earth plane when portals are opened (usually from Earth end) to the Astral planes.

Although they are flat, they are not technically entirely 2D as they can move in 3D. Most commonly they conform to essentially 2D surfaces (walls, floors, ground, etc) when in 3D environments. They can also 'bend' to go around corners or up from floor to a wall. They look like a shadow as it is cast onto something (has no volume like you would see on a foggy day). They have difficulty gathering energy while in 3D which is why they often seek out power sources including people to feed on. This is why they are considered 'negative'.