The Damnthing seems to know it was made out of the bits left over when they were finished with the cool monsters, and it has a resentment over it. From all reports the creatures don't even like themselves. Mating looks more like a fight to the death than even animal passion. They are never found in pairs unless it's a mother and young, and that relationship is short and not sweet.
The life span of the Damnthing has not been deeply studied (by anyone that survived to tell about it). Divination over the bodies of killed specimens indicates that none over the age of thirty have been found. It is believed they reach full growth by age 4.
What a Damnthing does have going for it is sheer toughness and animal cunning. Those that have tried hunting them, and returned to report, a smaller number that started, say the Damnthing will hunt you while you hunt it. Approaching one alone is a sure way to end up Damnthing fodder. For such a large creature they can be remarkably quiet when it suits them.
With no claws and a mouth ill suited to direct biting attacks Damnthings will rush anything smaller than themselves an attempt to overbear. They then bite at any limbs they can reach to disable and only then try to kill the creature. If crushing the limbs keeps the thing from moving they don't bother with killing it, they just start to eat. Large creatures are attacked with bites to the legs again. They can gore but don't prefer that attack unless the creature is at least as large as they are, or has no limbs in ready reach. Huge creatures they tend to ignore, unless they woke up on the wrong side of the lair, a frequent occurrence. A Damnthing will attack to watch you suffer and die, then never eat a bite.
The jaws of the Damnthing are powerful. In spite of the ill suited teeth they can crush the limbs of any creature size huge or smaller. Size small and smaller creatures are simply crushed. A crush can only happen with the bite. It will happen on any hit of an 18-20 the DM should pick a limb at random.
About the only thing that keeps Damnthings alive is their toughness. They are likely the second hardest creature to kill after Dragons.
You're shitting us, right? Sadly they are not. Agents in the Sauroi Lands have been shown these creatures, once deceased. From the above that is the best way to find them. Shoot early, shoot often.