Yep, just like GiselleX16 said (although I can't get the link in her post to work), it is totally random and rare. There are no specific items that will trigger a Gold Rune drop. Items that are very rare in the game and valuable to us as players are, unfortunately, not more likely to give Gold Runes than common items, like, say, Hides and Mermaid Scales. Therefore, I never send anything that I am not comfortable parting with, although my friend deadpixel said that she has amassed a lot of Rainbow Alicorns by faithfully fulfilling the Elves's requests every day. It is up to you how much you are willing to send through the Outpost. I have 4 accounts, all of which unlocked the Elven Outpost probably around 6 months ago. So far only 2 have received enough Gold Runes to get a Rainbow Alicorn. I went for at least a month without received gold runes on any accounts, then in one day 2 of my accounts each got 2 runes. It's totally unpredictable.