God Unit is a designation of various classes that are weak to specific legandary weapons.
The classification of "God units" in the Fire Emblem Series is fairly new, making its official appearance in Fire Emblem Echoes: Shadows of Valentia. However many Fire Emblem games do have major antagonists who are only weak to legendary weaponry.
Here is a list of dragon units:
Here is a list of skills that are effective to "God" units: