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 Mexican horror movie with innocent people who are chased by a monster who kills them for no good reason. But the twist here is that the people have illegally crossed the Mexico/USA border and the monster chasing them is a psycopathic American vigilante with a rifle and a dog trained to hunt and kill humans. All we know about the vigilante is that he tells his dog that he used to love this place but now he hates it, and that he drives a big pickup truck with a confederate flag on top, drinks Jack Daniels from the bottle in 120 degree heat, listens to old school country music, and he enjoys shooting illegal immigrants then screaming "You won't f**k with me now, will you!" But this movie is not only about illegal immigration, that's just the background to a tense, thrilling, and bloody chase movie. The people being chased are continually climbing up huge rocks and cliffs when they could be walking on flat land, just to create tension, so the filmmakers decided why not also throw in a random pile of rattlesnakes to ramp up the tension? My kind of movie.
The snake scene is in the middle of the movie. From up on a hillside, an illegal immigrant spots the vigilante's truck below. She bends over and crawls through the scrub so he won't see her, crawling right into a "nest" of rattlesnakes. (Seriously, it's the middle of summer and they want us to believe that there is a group of about a dozen rattlesnakes in an open area, rattling and lying and crawling around for no apparent reason. This is one of the stupidest snake myths, but we've seen it plenty of times before.) One snake crawls over the woman's legs but she calmly lets it crawl away and then avoids the rest of the snakes. That kind of realism warms my snake-loving heart, but I knew it was just setting us up for some snake action later. And at least the rattlesnakes here are not shown as bloodthirsty killers as they are in most movies. They make a nice contrast with the bloodthirsty vigilante, who cares more about his dog than the humans he hunts like animals.
The woman and Gael Garcia Bernal (who's the almost superhuman hero of the movie) decide to lure the vigilante away from his truck so they can run down and steal it. She puts a talking musical teddy bear that Bernal is bringing home to his son inside the "nest" of snakes to lure the dog. It works, but when the vigilante finds a snake crawling around his leg, he shoots it at close range then starts firing at the two who are driving away in his truck.
The snakes we see are probably Red Diamond Rattlesnakes. It's possible they are similar Western Diamond-backed Rattlesnakes, the rattlesnake most often used in movies, but since the movie was filmed in Baja California Norte and Sur, I assume they used local snakes, and diamond-backs are not found in Baja. The credits show two snake handlers, four members of a snake technical team, and the "Serpentarium Director." That's a lot of wranglers for a lot of dangerous snakes.
I don't know how they filmed the scene where a snake is shot with a rifle at the vigilante's feet, but it looks like a snake was actually shot or somehow exploded (it was probably a dead snake) so I assume the leg and foot we see in the shot were artificial. And the same for the scene where the snake crawls over the woman's legs.