To disable the animals and/or decorative skins that you've purchased, please follow the screenshots below.
Note that the last screenshot is intended to show you where the Disable buttons are located. You should click the buttons corresponding to the features you'd like to disable, which are not necessarily the ones called out in the screenshot (e.g., if you want to disable the Cats then you should click the button to the left of the Dogs button called out in the screenshot).
Note also that you should still get the 4% gold boost per animal purchased even if you disable a particular animal; the boost is dependent on the fact that the animal was purchased, not on whether it is currently enabled.
To re-enable any of the above items that you've disabled, return to the same screen and you'll see that the button says "Tap to enable." Click the button to re-enable your purchase.