Skullcrawlers

The Skullcrawlers are the tertiary antagonists in the 2017 adventure film Kong: Skull Island. They are monsters who lives in the Island and they are lead by Skull Devil aka. Big One, alpha of their kind.

Knowing the Threat
After the Sky Devils Team incident with Kong, Conrad and the survivors are in an old structure which is under the control of an ancient community. There they meet Marlow who tells them that it is not with whom one should fear but a few creatures called Skullcrawlers, monsters who responsible for the death of his partner and are looking to feed to square their neverending hunger. This explained why Kong attacked Conrad and co. because the giant ape aware that their actions will inevitably releasing surviving Skullcrawlers that he drove away from the island's surface, as his parents have gave their lives to stop the monster to destroy all lives on the island.

Jack Chapman, who was one of Sky Devils' commandates, is killed by one of them while he was distracted.

The Battle in the Death Valley
When Conrad and Skydevils arrive, they encounter one of the Skullcrawlers who murder Randa and several members of Sky Devils. During the battle, it is revealed that this Skullcrawler is the one who killed Chapman, and two members of the survivors are in open area that made them easily spotted by predators of the island. Conrad runs to save his friends in an attempt to kill them, San with a lighter manages to kill it.

The battle with Kong
When Marlow explains the history of the island, he mentions how Kong battles with the skullcrawlers, leading into a flashback showing a short battle between Kong and multiple sub-adult skullcrawlers, these ones being fairly large, almost as big as Kong himself. Kong eventually kills the group of beasts, as Marlow reveals that Kong is believed to only be capable of handling these sub adults.

The existence of larger Skullcrawlers eventually confirmed when a gigantic 95-foot Skullcrawler, which Hank Marlow identified as the "Big One," emerged from the water Packard and his men ambushed Kong in the swamp. The beast attacked the wounded Kong, who had just stomped Packard under his fist, but the humans were able to escape. By sunrise, the humans were attempting to rendezvous with Houston Brooks and Lin San, who were commanding the boat the Grey Fox. However, they were set upon by the giant Skullcrawler. One soldier, Earl Cole, attempted to sacrifice himself by detonating two grenades in front of the Skullcrawler, but the beast simply smacked him away with its tail. Kong soon arrived and smashed a boulder into the Skullcrawler's face, buying time for the humans to escape.

The creature then turned its focus to Kong, pouncing on him and gripping his throat with its jaws. Kong and the Skull Devil continued to brutally wrestle, pummel and bite each other until the Skull Devil forced Kong into a shipwreck with a blow from its tail, where the ape became entangled in the chains. With Kong subdued in the wreck, it returned its focus to the humans. As it approached the crew once more, the Skull Devil's right eye was destroyed by a flare that Weaver fired, and this allowed Kong to use and swing the propellor of a wrecked ship at his adversary like a medieval flail, which caught the beast in the shoulder and allowed Kong to resume his fight with it.

During the battle, Skull Devil was also fired upon by Conrad and his companions, allowing Kong to gain an advantage in the fight before finally uppercutting it with the ship's propeller, slicing the Skull Devil's throat wide open and seemingly killing it. As Kong saved Weaver, who had been knocked into the water earlier in the fight, the Skull Devil pounced at Kong once more. After coiling its tail around Kong, it wrapped its tongue around the ape's arm, pulling it down its gullet in an attempt to devour Weaver. After a brief struggle, Kong tore the creature's internal organs out of its mouth, killing it outright.

With that, the threat of Skullcrawlers presumably ends for good.

Behavior and Traits
Skullcrawlers are subterranean pack-hunting predators with mostly primal mindset. They are very aggressive as well as incredibly persistent in pursuing their prey. Even so, they are not instinctive monsters for two reasons. One is their intelligence that vary between one and another(with Ramarak the most smartest of all) and why they constantly hunt and devour creatures they come across: As result of their unusually heightened metabolisms, they suffer insatiable appetite that more than often drove them rabid in feeding frenzy as prolonged hunger would threatened to killed them from the inside, forcing them to constantly devouring unfortunate creatures they come across.

Enhanced Agility and Super Speed
Skullcrawlers can run extremely fast despite having only two legs, and their speed matches or surpasses that of an adult human, making it very difficult to outrun them. They can also make quick turns as well.

Burrowing
Being subterranean in nature, Skullcrawlers are capable in using their limbs to burrow deep into the earth.

Enhanced Durability and Stamina
Skullcrawlers' bodily tissues substantially hard enough to withstand bullets from rifles and the searing heat of a flamethrower. The Alpha Skullcrawler was only dazed when Kong slammed his head with a boulder or smashed his face with a tree branch. They also have incredible stamina, being able to stay in a full fledged fight for several minutes even whilst being incredibly injured.

Hunting Skills
Skullcrawlers are both solitary and pack hunters. One can kill a Sker Buffalo on its own, and they can cooperate to take down more powerful creatures such as Kong.

Enhanced Physical Strength and Combat
As the apex predators of Skull Island, Skullcrawlers are fearsome and deadly in battle, using their jaws full of razor-sharp teeth as their primary weapons. They can hold their own against opponents like Kong, able to withstand the blows and pummeling of the ape. Their serpent-like tails are used for swatting or for wrapping around an opponent during combat.

Prehensile Tail and Tongue
Skullcrawlers have prehensile tongues that they use to grab and quickly devour small prey, or for gripping large prey in combat. The tail is heavily muscled and extremely powerful, shown when the Alpha Skullcrawler lifted and flung Kong like a football across the lake with his tail. As stated before, the tail is also for wrapping around large prey, possibly to latch onto them for balance or to constrict them.

Weakness
Unfortunately, in spite of their great strengths, Skullcrawlers still have a lethal weakness: Due to their extremely enhanced metabolism, Skullcrawlers theoretically had to endure constant hunger and had they not devour at least one prey for too long, their body's metabolism will start feeding on their own bodies, slowly killing them in agonizing pain. This form of weakness however, merely implied and left to speculation as it's unclear how long a Skullcrawler had to endure the hunger before its own metabolism began to feed on its body. Although, it's apparent that many Skullcrawlers, especially smaller ones encountered by protagonists were under feeding frenzy that made them extremely aggressive due to the hunger. Even so, larger Skullcrawlers like Skull Devil, being the most experienced and oldest, had greater control over the hunger and self-control as it not taking reckless approach against Kong.

Trivia

 * Their design is influenced on the two-legged lizard from the original King Kong movie.
 * The skullcrawlers are similar to the V-rexs due to both being apex predators and rivals to Kong.