A prototype is a rough draft of a part for testing and further development before actual manufacturing. Prototypes can be low-cost fabricated or simulated using different techniques or even handcrafted.
Sheet metal fabricated prototypes. 
Sheet metal fabricated prototypes.
Extruded prototype.
3D printed prototype.
In some situations, designers may also create plastic prototypes for parts that will eventually be manufactured in metal. It is not very important if the part does not possess the structural qualities of the intended final part when it is only for visual evaluation.