From what I found through my search only harpies, weres, and centaurs can fertilize the eggs. Which doesn't really make sense, but there you go. I tested all three and now there is a harpy goblin, centaur goblin, and weregoblin running around.
Anyone able to tell me what scenes unlock the bottom three male goblin scenes? Those three, the final Urka, and the final Death Knight are the only scenes I can't seem to find.
Edit: Figured out the final Urka scene. You invite her to your camp, then fuck her. Either choice of position nets you the image.