In chapter 5, take a fish (BEFORE luring Gunther down from the cliff face), and go all the way back to the fan in the vent that blows upwards. Throw the fish onto the fan, and you'll get fish bones.
This will take a short delay, but unlock this achievement:
Keep the fishbones until the final game cutscene. You'll see the cat, and you'll be able to give them the fishbones for this achievement, straight after the 'boss' does his evil laugh.