From the Steam discussion forum.
1. A Duck and a Rabbit - You can find them on the Railstation Level (The first one)
2. A Sheep - You can find it on the Alps level.
3. A Puffin and a (Snow) Fox - You can find them on the Iceland level.
4. A Deer - You can find this majestic animal in the Forest level.
5. A Panda - You can find this cute creature on the Asia level.
6. A Koala and a Cockatoo - Those beautiful guys can be found on the Australia level.
7. A Seagull - This one will appear on the Beach level.
8. A Cow - You will find it on the last level - Poland