You get the ability to walk around with the name 'Hunter of ...' when you have killed 500 of that mob. (edited to add: not all mobs though, only gnolls, skeletons and fairies I believe in Qeynos, Freeport would have orcs instead of gnolls. Although both good and bad probably can have them all)
You get updates when you reach 200 and 300 I believe?
But you can check the amount of mobs you killed in North Qeynos just before the gates leading to Antonica, hard to target though. Can't remember where this can be checked in Freeport, sorry.
Edited, Thu Dec 30 11:26:57 2004 by Zieveraar