Unstable Thahds are the "upgraded" version of the thahd that can be accessed with a skill of 3 in Create Thahd. These were changed from the Thahd Shades in Geneforge 3. Unstable Thahds (and all unstable creations) lose heath over time and eventually die.
These are just like a normal thahds, just better, extra essence pumped into them, making them unstable (hence the namesake). They take less damage and hit more fiercely in combat. However, it is recommended to invest training into other creations due to the fact that these thahds do not last.