Teleport

Some monsters can be teleported to another island when they reach level 15. When a monster is teleported, they permanently leave the island they were born on and are moved to the new island, turning back into egg and going into the empty incubator on the destination island. This also resets the monster's level down to 1. This process cannot be reversed.

Gold Island
Gold Island is different from both Ethereal Island and Shugabush Island, monsters are not teleported to it. When any of the monsters (not including ethereal monsters) from Plant Island, Cold Island, Air Island, Water Island, or Earth Island reach level 15, that monster can be placed on Gold Island. Placing a monster on Gold Island does NOT remove the monster from the original island.

If you have a level 15 monster that you have placed on Gold Island, and then you later teleport the same monster to Shugabush Island, the monster will vanish from the original island it was from and from Gold Island.

For example, I have a Furcorn name Mr. Truffles on Plant Island. While lovingly taking care of Mr. Truffles and feeding him plenty of food, he reaches level 15. I place him on Gold Island. Mr. Truffles now exists on his original Plant Island home and Gold Island at the same time. Later, I decide to teleport Mr. Truffles to Shugabush Island. This causes Mr. Truffles to vanish from both his original Plant Island Home and from Gold Island. He becomes an egg again an appears in the Nursery on Shugabush Island.