All non-instance mobs have the potential to "leash" when you try to run too far. When a player moves too far from where the combat began, the mob will run very quickly back to where they were beforehand and evade all attacks until they return. It is possible to go a long distance with a target -- Kiting is more difficult in WoW, but can still be done to targets that you are able to keep prodding as you move. However, if you go long enough without adding additional aggro to the target, it will leash.
There are two main reasons for leashing:
Leashing was added to the game as an alternative to "training," a common concept in games like Everquest and Final Fantasy XI, where players would run until they zone out, leaving the enemies to slowly walk back. Not only does that create game exploits, but also players in WoW don't zone out...they just enter the next zone over and their opponents can follow them. A large portion of Everquest gameplay evolved around the act of leaving combat to split the enemies when that slowly staggered back to their home. Since Blizzard didn't want players to experience their game world through backdoor mechanics, they needed an alternative.
It is important to note that enemies within instances do not leash. They will follow you all the way to the exit or to your grave. If you do somehow leave combat, such as feigning death, they will leash and your group members will not be able to take advantage of them.