Bukkit

+ When "require_ownership" and "cancel_damage" are set to true, owned mobs can't be damaged by other players.

* "require_ownership" is set to true, leashes cannot be detached from fence posts by other players.

If you undefine defaultly leashable mobs they can still be leashed, this is by design choice, and may be removed in the future.

Leashes can be detached from fence posts when "require_ownership" is set to true by damaging the leashhitch or breaking the fence post, this is an unhandled event with leashes and will be fixed when the Bukkit Devs add this to the LeashAPI.