The description says: When first placed on an island, the Wubbox is a box that does nothing until you box (feed it) one of every 'regular' monster that exists on that island (except the limited edition monsters, the ethereal monsters and the legendary monsters).
I finally saved enough for a Wubbox for the plant island. But most of my monsters are level 15 monsters and only one of each exists. Question is can I first place the Wubbox then slowly breed 'new similar regular' monsters before boxing them. I don't wish to lose my regular monsters after working so hard to get them on gold island.
You can send monsters directly to the Wubbox from the incubator without having to place them on the island first. Another thing, it makes no difference if you feed the Wubbox a level 15 monster or level one monster, so best to breed all new level one monsters to feed the Wubbox.