You can create your own custom critters in the toolkit. You can also create your own encounters to specify a list of critters that will be spawned when a character enters a spawn 'zone' that you paint onto a map. You then select that zone and right-click on the spot where you want the critters to spawn. If you put critters with different levels of difficulty in the selection list for that spawn, the number and type of critters that pop up depends on the level of the character that first enters that zone to trigger the spawn. This can be very handy - especially for generating hunting grounds for players to pick up XP and loot between quests et al. Unfortunately, a level 21+ character generates the same number and type of critters as a level 1 character.
So, for example, if my spawn list says generate 2-4 critters and the critter list includes level 2 goblins and level 30 hostile monks, a level 40 player will always spawn 2 goblins instead of 2-4 level 30 monks as he or she should.
I assume this oversight is because Bioware only puts specific critters in their spawns for the Single Player Game (SPG) as opposed to a range of critters with different difficulty levels as many persistan world developers do. They just didn't notice because this was not really tested in the SPG.
I don't know when this will be fixed, but I don't really know when we will be going live either. I assume it will be corrected before we have any level 21+ characters saved in our online world, though. At least I sure hope so. We'll have some PO'd players if they finally reach epic levels and start spawning gerbils.