Named in the days before proper taxonomy, timber mantises are not, of course, mantises at all, and not even insects, but rather distant relatives of the common centipede. In a place too cold for dragons, these enormous arthropods have instead taken the place of massive apex predator.
Despite their great size, timber mantises are stealth predators, although their hunting behavior depends on gender. The larger females, ranging upwards of 50 feet in length, have a layer of green bristles that help them disguise themselves as trees, an illusion they complete by keeping their heads touching the ground, holding themselves up by keeping their bristly false limbs upon real trees.
Anything passing nearby will be grabbed at by massive clawed arms to be shoved into her horrible, grinding jaws. If she encounters another female of her kind if the forest, she will aggressively display until they either fight or one retreats. These fights are rarely lethal to themselves, but they are very lethal to the trees that happen to be in the vicinity of their combat. Were their enormous size not lethal enough, mixed in with their green bristles are agonizingly venomous needles. Some hypothesize that they were to defend them from an ancient predator even greater in size, as horrifying as that is to comprehend.
The males are much smaller, usually under 30 feet long, and totally lack the bristles of their female counterparts. Instead of burying their heads, they lurk entirely underground, burrowing underfoot to leap forth at prey. Goblins especially fear them for their skill at infiltrating goblin mounds and relentless hunger; many times did I hear of entire clans wiped out by a single male timber mantis digging into their home cavern.
Mantis courtship has never been seen, but there is no evidence that the female eats the male, as is often claimed. Whatever the case, after the deed is done, the female keeps her eggs within her bristles, where the needles camouflage and defend them. They stay with her until they are about five feet long, when her scraps are no longer enough to sustain them and they venture out on their own. While both sexes are born with bristles, the males lose theirs quickly to facilitate their chthonic lifestyle, becoming distinctly more reddish within a few years. By the time they are about five, members of each sex are far from fully grown, but ready to reproduce regardless.