Don't know if Turtle Rock said T5 is the last Tier or not, but the mutant monster form of Kala is too good of a opportunity to miss. The way I see it, there is the potential of having monster Kala as an additional monster to fight, and the way the characters themselves go on about what Kala has done, suggests that this is indeed a possibility.
Pseudo-Confirmed, a Kala monster was planned, but 2k canned the development of the game before it came to pass.
Monsters want to take over planets/kill life on planets. Monsters Evolve Goliath to surpass wild animals.
Monsters meet humans. Goliath is effective, but less so. Monsters Evolve Kraken to surpass human intellect.
Humans fight Kraken with mixed success. Monsters Evolve Wraith to bypass human cleverness by stealth, speed, and violence.
Humans fight wraith and learn how to deal with it. Monsters learn that they cannot avoid fights with humans, so they Evolve behemoth to take hits that can't be avoided.
Eventually, even the mighty behemoth is felled, countless new evolutions, and none working... And then the monsters put it all together. Instead of using specific tactics, they make their own version of a human. The Thing. And it may be still undefeated.
Nobody knows where they came from and how they were created. They could have been humans that were experimented on.
They're almost as smart as humans. Out of universe, it's due to a human controlling them. In-universe, it could be because they have a human intellect from their past selves.
All three of them are vaguely humanoid in appearance. This could also explain why the Wraith has a woman-like figure.
An interesting thought - maybe they've been merged in some way with the life on the planet? The Kraken and the Mammoth Birds both have a face full of tentacles, and both attack using bio-electric shocks...
the fourth tier hunters give another option. Hearing about Slim's history, you find out he's a third generation Basilisk soldier. Genetically modified humans meant for combat. The first generation were bigger with claws and tough skin, but were said to eventually go insane. The monsters could in fact be surviving Gen 1 Basilisk soldiers, now reduced to hunting instinct.
The outline of the fifth monster makes the above theory seem more likely. It looks like a giant humanoid bug. Slim is Gen 3, which means this one could be a Gen 1, who has lost its humanity. Also, tough skin? This thing is gonna have to be able to survive at least one orbital barrage, so it has to have pretty tough skin.
With Kala being revealed, it has become more likely. She has spliced herself with DNA from the Goliath, Kraken, and Wraith, becoming something close at an Eldritch Abomination. If she can splice herself with monster DNA, who's to say the monsters weren't failed splicing events.
Jossed, as the monsters came from another dimension. However a Hybrid between humans and monsters, the further mutated kala, was planned before 2k cancelled development.
And it is really not very fond of humanity trying to colonize it. So to kick them out, it starts cranking the hostility of the local flora and fauna Up to Eleven.
However, the hunters arrive, and they are extremely good at killing the aforementioned flora and fauna.
So enter the monsters, which are being directly compelled by Shear to kill the hunters above all else in order to remove the one element capable of resisting Shear's incredibly hostile environment.
Jossed, as the monsters came from another dimension. That said, humanity's method of FTL travel wound up causing massive destruction in their home dimension, which is what caused the monsters to invade Shear and Factor.