Creekbank Valley allows you to capture new pets. When you first enter you will be on an island in the center of a small lake, with three different environments surrounding the lake. Each environment contains creatures of a different type; one has Wind Puppets, one has Khonem and one has Scarabs. When you attack and kill one of these creatures they have a chance to drop a Creature Soul of that pet type, which you can capture. These creatures do not fight back, but they do try to run away from you when you attack them. There are also traps scattered around the three areas which need to be repaired (done by clicking on them and waiting a few seconds). Once activated, the traps periodically send out a small shockwave that will temporarily snare a creature and prevent it from running away.
You should coordinate with your teammates and pick only one area/pet type to go for in the beginning, for the following reason: Once ten creature souls of a single type have been captured between your teammates, a Boss creature of that type will appear close to the lake in its corresponding environment. Unlike the rest of the creatures, this one does fight back. Killing it also gives a chance for a Creature Soul to appear, and this one has a high chance of being a Blue-class pet. They also drop between 1 - 3 pet skills. Once 10 of each Creature Soul has been captured and their respective Boss defeated, a fourth and final Boss Creature will appear.