Entities

Entity polys extend EntityPoly. This poly is basically just a factory for a Wizard. Wizards are explained in greater detail here, they allow you to summon other vanilla entities to represent your modded one and can also do other things like sending particles. It's recommended to extend EntityWizard instead of Wizard to make your life easier.

For an example, check out MissingEntityPoly, which is the default entity poly. It simply spawns a barrier item to let people know that there's an entity there but PolyMc can't display it.