Yes, it is possible (hey, this is FlightGear). The question is how one would do it.
I think that Marc Kraus have a scenario where you would drive horses with a helicopter in his
Lake of Constance hangar.
Another way could possibly be if one could "sprinkle" cows over pastures by adding them to the terrain material definitions. I am not sure, but I guess one could even use an xml file with animations of them gracing the fields, though it could be a performance bottleneck and it could look "odd" if they wander around near water or cliffs.
That would be great for scenarios set at mesas or islands in the north of South America. Imagine for example downed pilot rescue, but with a twist.