i'm assuming this one!
"
Unless specific permission is given, killing or stealing mobs that are "owned" by other players is considered griefing. Mobs are considered owned if they are in clearly marked and enclosed areas. This applies to hostile mobs if and only if they are contained so that anyone in the area can not be harmed by the mob (e.g. a skeleton should not be able to shoot players walking by). Also, players are not allowed to kill a horse being ridden by another player. Mobs found in the wild may be killed or captured."