There are very many possible answers. For example a polyhedron with 18 faces in the form of a drum. The skins are octagonal, and the side of the drum is made up of triangles, alternating pointing upwards and downwards.
Octagon. * * * * * An octagon is 2-dimensional. The regular 3-d shape (all faces the same) is the octahedron, but a hexagonal prism also has 8 faces.
An octagon
A shape with 8 faces is called an octahedron. An octahedron is a polyhedron with eight faces, each of which is an equilateral triangle. It is a three-dimensional shape that is commonly used in geometry and mathematics.
If its 2-dimensional then its an octagon, if it is 3-dimensional then it is called an octahedron.
Since a cube has 6 faces, you would be looking for a solid shape with only 2 faces and no such solid exists.
The question is ambiguous. An octagon is a 2-dimensional shape with 8 edges and 8 vertices. Does a 3-D octagonal shape mean one with 8 edges or 8 faces or vertices, or faces which are 2d octagons?
triangular prism
a triangular prism
An octagon based prism.
rectangle
An octagon is a two-dimensional figure. It has no faces but does have 8 sides.
A cone perhaps