[1.2.198]Goldrich Event, Sleepywood Expansion?

The ETC eggs give EXP when you bring them to Goldrich.

And Isaac, any ideas on what this does?

It has a quest script where a Zombie mushroom-type seal appears on it, like when the Snow Spirit's statue is reconstructed after you finish that quest. It's undoubtedly related to the Balrog Boss, possibly a quest for defeating that.

For the Goldrich quest maps:
I think that people with lvls 13-20 are warped to map #1,
21-30 to #2, 31-40 to #3, and so on.

Can you check if there are datas for 12 different "Golden pigs" in the wz files?

this would explain the 12 different eggs(one for each level range)

If there were 12 gold pigs it would be mentioned. There are 12 copies of the eggs though, so more than likely the drop is related to some quest that you have active and that's how it varies for level ranges.