Some animals are animated and will look like they are eating your crops, don't worry they don't!
If you prefer that they do not wander around the farm and would like to keep them within something like fences when decorating your farm. You can add a check mark to a setting in your preferences to stop them from moving around by themselves. This applies to all of your farms, it does not apply to a specific farm. Change the setting refresh the farm then move the animals to where you would like them to be within your fences.
To freeze your animals click on the preferences icon top right of your game screen and add a check mark to Freeze Animals, then click on the green check mark at the bottom of the window to save your changes.