Leveled creatures change, depending on your level. When you enter an area, the game checks your level and pulls a creature from the list that is less than or equal to your level. In some instances the game will either add 2 to your level or subtract 2 from your level before consulting the list. If your level is lower than the lowest level of creature on the list, then no creature will appear.
Level 1: Corprus Stalker,
Ash Slave
Level 8: Lame Corprus, Ash
Zombie
Level 15: Ash Ghoul
Level 25: Ascended Sleeper
Level 3: Corprus Stalker,
Ash Slave
Level 10: Lame Corprus,
Ash Zombie
Level 17: Ash Ghoul
Level 27: Ascended Sleeper
Level 1: Corprus Stalker,
Ash Slave
Level 6: Lame Corprus, Ash
Zombie
Level 13: Ash Ghoul
Level 23: Ascended Sleeper