So far I've got a harpy-goblin,were wolf-goblin and centaur-goblin. I tried the fish-goblin, but only ended up with the fish (mermaid). Tried the wasp-goblin, but no joy. Just got wasps out my butt.
Best way seems to be get pregnant with the Goblin (ask her to impregnate you) and then get fucked by another creature. the pregnancy doesn't always work though. Sometime I get a message that the eggs passed out (similar to cum).