I notice that idle animals will tend to go to the meeting hall.
So, lets say there is a Ranch zone you can define, either for all animals or a specific kind. Animals selected should tend to go there, much like they currently do to the meeting hall. Further, once in such a zone, the pathfinding cost to step outside of it should go up (not by a huge degree, but enough to make them prefer to stay in over going out).
Changing the pathfinding cost doesn't do that.
praguepride's major complaint, which I think has some merit, is the processing cost of having large numbers of tame animals (who don't really have anywhere to go) trying to pathfind all the time.
So if there's a zone like this, I suggest that docile animals (defined by a tag) put into this zone should turn off pathfinding and just wander randomly. (You still need a fence to make sure they don't wander out of the zone.) Note that, once animals require food and water instead of being a massive cheat, they should be smart enough to pathfind to that, but it will have to be inside the fence.
But then my war grizzly's are trapped!
DF doesn't have war grizzlies. You must be thinking of something else.