Right now the only way of herding animals is to send them all to the pen, which is a hassle when you want many interactions with one type of animal, and especially one single animal.
I propose 'the dog', a simple animal with the verbs roam, sit, and feed.
The roam command will make the dog patrol the yard. He moves in one direction until he reaches a cliff or barrier, and continues moving in the other direction. Using the roam command on an already roaming dog will change his direction .
The sit command makes the dog stay in place, and other animals will not pass by the dog! This would allow piggies, chickens, and butterflies to be separated into their groups by species or any other manner. It could also help prevent piggies from eating from your trees without being confined to a pen.
The feed command is the only way to keep the dog alive. The carnivorous dog will not eat from any of your plots or trees, so you must feed it meat or meat-based meals. If you do not feed the dog, it will eat one of your animals!
All of these suggestions are meant to 'kill two rooks with one orb': provide another animal for the people who want more animals, and provide better herdkeeping for the people who want more organized yards.
Other thoughts:
Maybe the dog can eat from eggplants?
Maybe have a pet verb that allows you to stroke the dog for a mood boost?
Should patrolling dogs obey or ignore sitting dogs?
Should remote herdkeeping affect dogs?
I'd love to hear feedback on this!