Zoo Tycoon

There are 101 animals in the game to release into the wild. This includes both animals that can inhabit exhibits and mini exhibits. To release an animal into the wild, you need to level it up to level 15. The fastest way to level an animal is to maximize its happiness.

To unlock all of the animals, you need to get to level 40 in challenge mode. There are certain animals that are specific to challenge mode. I found the fastest way to do this was to work on challenges, and to upgrade everything once I started having more than enough money to do so. I got stuck around level 35 and wasn't really getting much more xp, so I just bought like 5 decorations and like 5 food exhibits and just upgraded them until I leveled up, then repeated the process until I was level 40.

Once you hit level 40, you unlock the last animal. I found the easiest way to keep track of which animals I had released was to not research them until I was ready to level them up, but you can always go to the pause menu -> zoopedia -> animals -> [animal type] -> [animal name] and look on the right side where it says "released". If you've released the animal, there will be a check mark next to it.

I would recommend working on this while working on the breeding achievements, but if you want to get this one as fast as possible, pick a type of animal, and put as many different animal of that as you can in one exhibit.

Ex: All of the Chimpanzees are happiest in the Tropical Exhibit. They all eat the same food, and play with the same toys. So you put one of each of 2 chimps and then 2 of a third type of chimp in the same Large Tropical exhibit (you can only have 3 different species in one exhibit), put a fruit and a vegetable feeder, a shower, and 2 climbing toys, and it will max out their happiness. And because you have 3 different chimps (and 4 chimps total), it meets the social needs as well, but allows you to work on 3 different animals, instead of having 4 of the same chimp to meet the social requirement, but only working on one animal.

You can do the same thing with the mini exhibit animals.

Ex: 3 different types of Macaw in the same exhibit.

You always want to wait until all animals in an exhibit are level 15 before you release them, unless you are going to replace the animal with another one, or you won't lower the other animals' social needs by releasing one.

And obviously, adopting the highest level animals helps speed this along!

No real trick here, just unlock all the animals and it's grind time. Level 15 makes them able to be released into the wild. You can check the status of what you have already released in the zoopedia and courtesy of IGN you can have a checklist for the rest:

Lions (7) – Asiatic, West African, Transvaal, Masai, Barbary, Congo, Katanga
Tigers (6) – Bengal, Sumatran, Siberian, Indochinese, Malayan, South China
Bears (13) – Grizzly, Kodiak, Himalayan Brown, Tibetan Blue, Syrian Brown, Gobi, Olympic Black,
Kermode, Polar, Cinnamon, Glacier, Formosan, Himalayan Black
Elephant (6) – African Bush, African Forest, Indian, Sri Lankan, Sumatran, Borneo
Rhinos (8) – Southern White, Black South Western, Northern White, Eastern Black, South Central
Black, Indian, Javan, Sumatran
Hippo
Giraffes (9) – Reticulated, Masai, West African, Angolan, Rothschild, South African, Thornicroft, Kordofan, Nubian
Chimps (5) – Central, Western, Nigeria-Cameroon, Eastern, Bonobo
Antelope (10) – Addax, Lowland Bongo, Gemsbok, Nyala, Roan, Topi, Sable, Scimitar Oryx,
Bushbuck, Pronghorn
Komodo Dragon
Nile Monitor
Perentie Lizard
Lemurs (4) – Ringtailed, Red, Ruffed, Red Ruffed
Capuchins (4) – Black, Golden-Bellied, Tufted, White-Headed
Binturong
Red Panda
Sloth (2) – Brown-throated three-toed, Pale-throated three-toed
Mongoose (2) – White-Tailed, Banded
Fossa
Meerkat
Iguanas (2) – Lesser Antilean, Green
Reticulated Python
Boa Constrictor
Yellow Anaconda
Flamingos (3) - Greater, Lesser, American
Peafowl (2) – Indian, Green
Galapagos Great Tortoise
Macaws – (6) Blue and Yellow, Great Green, Military, Blue Throated, Scarlet, Hyacinth