Here is my experience of the ranks and titles...
The mobs which can give you a title are gnolls, orcs, goblins, undead and fay (oft referred to as fairies. In both Freeport and Qeynos is a notice that when clicked will tell you how many of each you have killed. In North Qeynos by the Claymore Plaza on the Antonica side is a lampost. You can interact with the wanted notice attached to it.
Rank/Title Number of Kills needed
Gnoll Killer 25
Initiate Gnoll Hunter 100
Accomplished Gnoll Hunter 250
Master Gnoll Hunter 500....
....shows title as Hunter of Gnolls
Initiate Gnoll Slayer 1000
Gnoll Slayer 2500
Master Gnoll Slayer 5000....
....shows title as Slayer of Gnolls
I can confirm up to Slayer....but unsure how many kills needed to make Destroyer of ....., it may be a natural progression with 10000, 25000 and then 50000 for the title of Destroyer
Tikketyboo, Hunter of Fairies
Cootchycoo, Slayer of Goblins