Prefabs and Modding

I'm trying to make a game that is easy to mod. I've read about loading DLLs at runtime which looks reasonably straightforward but I'm wondering how this is meant to work with prefabs. As I understand it, prefabs can only be made in the editor. How is a modder supposed to create a prefab only with a DLL? If that's actually not possible, how would modders make, for example, a new monster at runtime? Is there a way of doing it without the use of prefabs? I understand text and textures can be loaded at runtime but how is the modder meant to be a monster that could be spawned several times they way they can using prefabs?