Happy Aquarium: Guide to the new "Rescue an Animal!" feature
Players will encounter it randomly, but once triggered, it operates in the same way as the Monster Expeditions. Because you'll need friends to help you procure a certain number of items to free the trapped creature.
If you choose to rescue the animal, the barrel will appear in your tank. Click on the barrel, and you'll get a window showing you the items you'll need. Each barrel requires the following: 3 Helps, 3 Plyers [sic], 3 Crowbars, 3 Screwdrivers, 3 Hammers, and 4 Gloves. And if you're friend-less and rich enough, each item of help can be bought for 15 Pearls.
When all the items are collected and the animal is ready to be released, you'll see the words "RESCUE NOW!" on the barrel. Click on that, and the creature is yours! Now if only we knew what this mysterious creature is...
UPDATE: ...the mysterious creature is a White Beluga!