The monsters are this without a doubt. They are massive predators far beyond anything on Earth or Shear, capable of ripping apart fortified bunkers and starships. They can smell you from hundreds of meters away and will relentlessly hunt you down, slaughter you, and feed on your corpse. To make things worse, not only are they immensely powerful killers, they are smart. One can only imagine the colonist's terror as they realized that the monsters were intelligent, luring them into traps and countering their tactics.
The Goliath is the simplest of the five, but no less terrifying for that. It doesn't have fancy powers like the others so it has to maul you to death with its bare claws. Or just burn you to death and bludgeon the smoldering corpse with a boulder.
The Kraken is a mini Eldritch Abomination. Despite its size it darts around in the sky, hurling lighting and dark matter.
The Wraith is like a monstrous ninja, appearing out of nowhere and snatching you away from your team.
While the Behemoth seems like an armored giant, its exposed organs and unusually high pitched shriek just seem wrong. This isn't helped by the way you occasionally see its real head underneath the armor plating, which seems slimy and malformed◊.
The Gorgon is perhaps the worst of the bunch. It's body is just... unnatural, not fitting into any traditional type of animal. Add to that the way it sprays acid, traps you in venomous webs, and its habit of hiding high up cliffs to drop down on you and you have a living nightmare.
One that never made it into the final game: The Host. A massive, decayed looking insectoid monster brimming with parasitic creatures that it uses as its weapons. Its concept◊ art◊ is unnerving, seeming like a collection of mutations and deformities. The finished product◊, while pretty cool, wasn't much better. It got as far as early tests of the game, allowing us to see it in unnerving action. And according to Word of God, it's canon.
The Hybrid, the end result of Kala's mutation. It a monster with all of Kala's intelligence, as well as the ability to use the powers of any other monster. The fact that she was incapable of leaving Shear is the only thing that stopped humanity from being overwhelmed.
The machine-monster. It never appears in-game. It never gets an ability reveal. It never even gets fully described. But it's terrifying despite all that because of the implications: the monsters aren't limited to biology. It grew metal armor, AI cores, data cables, and more, all merged with flesh to create a horrific abomination. To make things worse, it can take over the minds of any AI it comes in contact with. The monsters can grow inorganic materials, even extremely complex machinery. That's humanity's technological advantage out the window.
All of the above are only a few monster species. Canon stories teased three others and there's quite a few pieces of concept art for unused monsters. A fan counted them all and assuming the species shown, concepted, and teased at are the only types, there are twenty-six different monster species.
The monsters may in fact be literal nightmare fuel, deriving their forms from the subconscious fears of humans. This is still preferable to the alternative that their prescience is what instilled those terrors into primeval human subconscious.
The process of evolving has been described by Word of God as the most painful thing a living creature could experience, something no human could hope to endure. Yet the monsters willingly go through it, even twice, so that they can more easily kill the hunters and settlers. Exactly what is driving them to put themselves through so much pain? Base instinct? Or an abject need to obliterate the human race?
Kala and Caira have a discussion about how Kala managed to graft monster DNA to her own body. Kala explains that she didn't get anywhere at first. And then she applied an in-universe equation and solved the problem. Said equation has nothing to do with biology, but is instead tied to physics. Caira's reaction is at first a Flat "What", followed by a small freak-out over what this implies. She then later discusses this with Lennox, adding that said equation is both poorly understood and the basis for human FTL travel. If that's not the hallmark of an Eldritch Abomination, then what is?
Some of the monster fan art can definitely stray into the unnerving.
Several of the hunters have this included in their backstories. Maggie was trapped alone on a planet full of monsters for over two years. Torvald was ripped apart and disemboweled, leaving him just alive enough to save. Slim went through a four year long war that left him traumatized to the point where he can't remember his own name and involved ' hurtling naked through deep space'. Jack went through the same thing as Maggie at the age of 16. Kala fused herself with monster HNA, causing her to hear their thoughts and slowly overwhelming her own DNA until she transforms completely.
Part of the backstory of Evolve is the Mutagen Wars, a series of rebellions by the Basilisk Nebula using mutant soldiers to compensate for their lack of resources. The first generation were described as hideous monsters, immune to bullets, and driven completely insane due to the unstable process. Even if they had won, that would have caused a severe problem when their former masters tried to bring them back under control.
Often overshadowed by the monsters, the various flora and fauna of Shear would be this to the colonists. There are plants that will eat you alive, car sized beetles, venomous dog-lizards, shark-crocodile-dinosaurs in almost every sizable body of water, and aquatic predators that would best be described as sea monsters.