A block similar to the barrier block but for mobs only.

One of the many challenges to Minecraft is making a city or town more lively. One of the ways this can be done is by filling it with mobs such as villagers. The problem however is that mobs like to path find and wander off. This leads to areas being over concentrated with mobs and void of mobs in other areas. Sure you can just use barrier blocks to keep them in certain areas but then they become restrictive for the player. If there was a block like the barrier but for mobs only it would allow the player to roam freely but mobs would be constrained. That way one can liven up a city or town but not have to worry about constraints on the player or have all the mobs wander off. Now there are ways to block pathfinding such as rails but they stand out. But with a block like the barrier or the light block a player can block the mobs from pathfinding but not have to worry about how it looks. One of the trade offs is that one can only access it in creative mode. because of this I can see it being used mainly by map makers. I think it would be a great block for map makers to make their worlds more lively.