Release one of every animal type into the wild.
This achievement requires that you release at least one of every animal type into the wild across all of your zoos and can be done in any game mode. To release an animal into the wild, it must be Level 15. Animals increase in level as they age from Level 1 when they are born to a maximum of Level 15. Depending on their level of care, it will generally take around two hours of actual playtime for a single animal to go from Level 1 to 15. Once an animal is Level 15, go into Edit Exhibit>Animals>View Animals. Use the bumpers to highlight the Level 15 animal and then to select it. Choose Release to Wild and the animal will be removed from your zoo (you do not get anything). Do this for all 52 types of animals (both exhibit and mini-exhibit animals) and the achievement will unlock. Color variants are irrelevant for this achievement.
The larger main-exhibit animals will be released as part of the breeding process for the "One of Everything" achievement. The mini-exhibit animals should be adopted at as high of level as possible and then aged to Level 15 to be released.