If you want to teleport for example the Zombie which was most recently added, you would do "/tp @e[type=Zombie,c=-1] ~ ~1 ~". Summon a few more Zombies and if you run the tp command again, it wom't tp the most recently added Zombie. Same with c=1 (the oldest entity).

Steps to reproduce:

• Do /gamerule doMobSpawning false
• Do /kill @e[type=Zombie]
• Summon four Zombies with the CustomName 1, 2, 3 and 4 in exactly this order. ( /summon Zombie ~ ~ ~ {CustomName:1}

  )

• Do /kill @e[type=Zombie,c=-1] 4x times (This should kill the Zombies in this order: 4, 3, 2, 1)
• Repeat
  You will see it doesn't kill the Zombies in this order.

Do this again in Snapshot 14w06a and the Zombies get killed in the right order!