The Trebhum are so good at surviving because they are small omnivores with generalized diets. Other species, being more specialized, are less suited to survive a sudden cataclysm.
The high concentration of Tonglegrops in the latter part of the Savanna may bring to mind the issue of More Predators Than Prey, but it's entirely justified: adults are omnivores that include plants in their diet, and young are herbivorous, allowing the ecosystem to support a greater number of them than what would be feasible if they were purely meat-eaters.
Some mutations have negative effects that make the Trebhum blind or legless, for example. This is actually accurate for mutations in evolution: some mutations make a creature more likely to survive in its environment, while other mutations hamper its survival.
Zooshgargs are among the only creatures that don't flee the Cylinder: not only because they fly so high— but also, because animals fleeing the Cylinder make easy prey, like predatory birds that pounce on rodents fleeing bushfires.
Tripoboosh and Omnogroms still attack the player even when the cylinder is rolling, unlike most other animals that drop all aggression and flee for their lives. That's explained in lore in that these two species both have very poor eyesight, and are also dim as a sack of bricks, meaning they may not even know what's happening until it's too late to escape.
It's odd that Omnogroms have disturbing human-like teeth, despite being predators: however, blunt teeth are an adaptation to durophagy: animals specialized to eat hard foods like armored shelled prey and bones from carrion.
One old folktale from the Trebhum culture, which sadly has been lost save for its title, is "The Tripobosh and The Onogrosh Go Hunting Together". As Tripobosh are tundra-dwellers and Onogrosh are desert-dwellers, it likely didn't go very well in the end.
Why were the Grashtuubs seemingly aasisting the Servants of the Cylinder? Because they eat Celestial Trewhaalas: and their favored prey are currently the Servants's enemies. They're sticking around hoping to get a meal in the bargain.
Fridge Logic:
So the Eternal Cylinder is crushing a whole planet. Is the planet a Flat Earth a la Discworld? Is the Cylider curved or maybe donut-shaped?
If simply jumping over the cylinder is a viable option then how come flying creatures like Onkifurts are still killed by it?
The Onkifurt might not be able to fly over the Cylinder because it can't inflate itself enough to gain the lift needed to get over the Cylinder.
Somehow, Trebhum with the cube body mutation are still able to roll around.
How is the Tonglegrop's acid sac able to roll independently of its body? That implies it's not connected to the creature's body at all, which raises the question how it even is used in storing acid for its spit attack.
If Trewhaalas are not native to the Trebhum homeworld then why are there animals specialized for eating them? Are the Grashtuubs non-native too?
No Biochemical Barriers mixed with opportunism? It only says that they eat a list of things that include the Celestials.
Tonglegrops are pink in the Savannah and yellow in the Desert, implying that this species difference is due to camouflage...which would be sensible if both species didn't have a huge glowing green acid sac that can be seen from far away.
Fridge Horror
All the elders are inevitably crushed by the Cylinder whenever you leave their shrines. And the towering Great Gaaarhs too.
Given you hatch new party members, and that the Trebhum are tiny compared to the elders, these are literal infants being forced to survive by themselves in a post-apocalyptic world.
The acid-spewing Tonglegrops hunt the smaller pink creatures which according to lore are an immature larval stage. The Tonglegrop is devouring babies of its own species!
The Trebhum were once a highly-advanced species. And yet Treghrums, creatures that mimic Trebhums, still exist, implying that Trebhum society was once infiltrated by murderous monsters that looked just like them...
More of Fridge Squick, but the Trebhums, and other ingested particles, end up inside the gaseous sac on the Great Gaaahr's back, and leave through a valve on the side. Which suggests that the Gaaahr's dorsal sac is actually its external digestive system, and the pink bombs that drop off of it are actually poop.
