Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a science-fiction/action film that uses a snake in an unusual way that actually advances the plot in a very interesting way, though I suspect it might be a bit confusing to some of the audience. The film is set in the northern Great Plains in 1719 and focuses on a young Comanche woman named Naru (Amber Midthunder). (Unfortunately, most of the action is set in a wet boreal forest, not the Great Plains, and the people are called Comanche, a tribe which is found in the southern Great Plains, but it's easy to forget about those errors.) Naru is trying to prove to her brother Taabe (Dakota Beavers) and the other members of her tribe that she is a skilled hunter, so she doesn't have to gather plants like the other women. She wants to be a predator, but she becomes prey for the Predator. But then the Predator becomes her prey.
This is the fifth film in the Predator franchise which started with "Predator" (1987). The Predators are extraterrestrial trophy-hunters who hunt humans and other animals that can fight back, to keep their skulls and other parts as trophies. They don't hunt unarmed or disabled people or children. They hunt using laser-guided weapons and they will use explosives when needed. One of their biggest advantages is the ability to become transparent, blending into their surroundings to become invisible to animals with normal eyesight, such as humans. A Predator's main vision is infrared thermal vision, which we see on screen in bright colors, with blue being the coldest, and red the hottest. The transparency and thermal vision are good to know about to understand the snake scene.
The snake scene happens before we know much about the Predator. From a very low vantage point down in the duff on the forest floor, we see an ant crawling up into what appears to be thin air. Then we see some ripples in the "air" and we realize that the ant is crawling on the transparent Predator. A mouse reaches up and grabs the ant in its mouth, then turns to the left to eat it. The camera pans to the left past the mouse then focuses on a rattlesnake coiled up nearby. The snake quickly strikes at the mouse, grabbing it in its mouth, then crawling to the right behind part of the invisible Predator with the mouse in its mouth. Behind the Predator, the snake becomes a blur to us, then as it crawls out from behind the Predator we see it in full focus again. It becomes agitated, drops the mouse, rears up in a defensive stance, then strikes out to the left with its mouth open and fangs ready. But before it hits what it's aiming at, two sharp blades stab the snake in the head then lift it up, again behind the invisible Predator where it is hard to see exactly what is happening. It appears that the blades pass down either side of its body, effectively skinning it, and then the snake appears to explode. Then the camera cuts to another scene. The whole sequence is less than a minute long.
Like the Predators, rattlesnakes have heat sensing pits on their cheeks. (That's why they're called "pit vipers.") The snake sensed the heat of the Predator's body, then dropped the mouse to defend itself from the invisible threat. The Predator, sensing that the snake was capable of fighting back, saw it as suitable prey and killed and skinned it.
We don't see what it took as a trophy, but a few minutes later when Naru is with a search party at night, her dogs finds a dead mouse. Then she finds the rattlesnake's body nearby. It has been skinned and apparently gutted. All we see is the flesh and the rattle. The body then moves as if its still alive, jump-scaring us and Naru as well.
The Predator probably took the snake's head as a trophy. The camera later shows us the snakes skeleton up on a tree branch, and it's also missing the head. When we saw the snake explode earlier, we probably saw its skin and guts flying out of its body, as the backbone was flung up into the tree.
When Naru finds the mouse and the dead snake, she knows something is very wrong. Predators don't abandon their prey for no reason. She tries to warn the hunting party, but nobody is concerned. She asks her brother what would skin a snake like that, and later she reminds him that whatever it is they're looking for skinned and gutted a snake, for apparently no reason, but it takes more evidence before anybody takes her concern seriously, and by then it's too late for them.
There is no real snake in this movie - all we see is a CGI rattlesnake (and mouse). As they often do in movies, the lighting is kept dark to more easily fool us, and the snake is blurred when it is behind the Predator which helps to obscure it even more. But overall they did a good job on the snake. Most people wouldn't know it's not real.