Frogs, foxes, butterflies, birds and others are decorative animals, and they visit the farms every now and then. They have no particular aim and they do not threaten the farm in any way. Some decorations may increase the possibility to have more of them (just check the description of the decoration), but unfortunately we cannot put these animals in the farms or take them away.
Please, also notice that these animals need free space in order to move, so if your farm is a little crowded they may be stuck and unable to move from their spot. You may want to consider storing some decorations and rearranging the farm's design to leave them more free space.
Some decorations may increase the possibility to have more of a certain type of animal, for example adding a pond will increase the number of frogs that visit your farm and a birdhouse may attract more birds. Tap repeatedly on the decoration to attract your desired animals.
You can check the description of the decoration for more information.