The first film to show dinosaurs and cowboys in the same picture, it is notable for being based on a story idea by special effects innovator Willis O'Brien. It used a form of stop motion called replacement animation to bring the dinosaur to life. O'Brien co-wrote the script under the pseudonym El Toro Estrella; O'Brien was also to have originally done the special effects for this movie, but this did not happen for reasons unknown. Jack Rabin, Henry Sharp and Louis de Witt ended up doing the effects, probably based on O'Brien's storyboards. This film was one of the few American/Mexican co-productions of the 50s. It was made in color and Scope. It starred Guy Madison and Patricia Medina, and was produced/ co-directed by Edward Nassour.
In southern Mexico at the turn of the 20th century, cattle and farmers mysteriously disappear at a location called "Hollow Mountain". The mountain has never been explored and the swamp at its base is said to claim the lives of anyone foolish enough to go to its banks. In spite of these tales and possible perils, American cowboy Jimmy Ryan leads three cowboys into the area in search of lost cattle. When they arrive they find mysterious tracks and believe the curse from Hollow Mountain is responsible. Whilst trying to track the curse down one of them falls into a tar pit at the base of the swamp and nearly drowns, but is rescued.
Back in town, Jimmy meets a Mexican boy, Panchito, and his father Pancho, who own a large ranch not far from Hollow Mountain. As the two are leaving to check on the cattle, a group of children throw firecrackers at them, causing Pancho to fall off his horse and get dragged across the ground. Jimmy notices this happen and stops the horse saving Pancho. He begins falling in love with the beautiful Sarita, who had also stopped to help Pancho. Jimmy and Sarita head to the cafe of Don Pedro, an old Mexican who Jimmy talks to about the disappearing cattle. While the two are talking, Enrique, a tough old Mexican, comes by. Enrique does not want Jimmy to ranch his cattle here. The two almost fight but Don breaks it up.
Jimmy and Felipe read a note from Sarita saying she is out at the ranch checking on the cattle. Jimmy and Felipe hire Pancho and Panchito, and together they head out to the ranch. At the ranch Sarita is disappointed to hear that the Panchos have abandoned life in her cottage and have come to work for Jimmy instead. She tries to persuade them to leave, but they both refuse.
Sarita goes to the top of Hollow Mountain to see Jimmy again, and the two share a romantic moment together. Jimmy becomes jealous when Sarita says she will me married to Enrique in two weeks and is surprised to hear from Sarita that Enrique is actually quite nice. When she leaves to go home, she finds her horse is missing. Reluctantly she agrees to ride Jimmy's horse with him back to town.
When they arrive, Enrique spots them. Enraged at seeing Jimmy hanging out with his betrothed, he attacks him. Eventually Jimmy wins and is able to overpower Enrique, but is scolded by Don. Don tells Jimmy that Enrique wants to buy his ranch. But Jimmy is determined to stand his ground no matter what and refuses. Don says that friction will continue between the two if Jimmy does not give up the ranch before a new shipment of cattle comes, but is forced to retreat when Jimmy still refuses.
A few days before the wedding of Enrique and Sarita, Enrique still fumes about Jimmy and what he did with her. Sarita pleads for him to become friends with Jimmy but he refuses. Back at Hollow Mountain, Jimmy and Felipe lead the Panchos to their cottage, whose former owner mysteriously disappeared. While Panchito guards the horses, the three men head out and find the body of another missing cow. Pancho wants to explore the swamp but is stopped by Jimmy, who feels it is too unsafe for him. In town Jimmy stops to greet Sarita, who apologizes for her rude attitude towards him in their last encounter. Yet again, Enrique spots them and cusses about Jimmy again. But this time he has a new plan in store: he sends out two henchmen to attempt to steal some cattle while Jimmy and Felipe are away.
Meanwhile, Jimmy gets mad when the manager of the town says that a new shipment of cattle cannot come. The manager says that if more cattle come, there will be more people trying to convert his property. Jimmy agrees to this and leaves. As he leaves, he finds Felipe has hire two men not knowing that they are spies sent by Enrique. Later Jimmy visits Pancho's house and while feeding a calf gets a letter from Sarita to meet her at the graveyard.
At the ranch, Pancho asks Panchito to wait for him at the cottage while he goes to look for the lost cattle. Panchito, aware of the curse said to exist in the area, pleads for his father to stay there, but he says everything will be all right and then leaves. At the graveyard, Sarita is relieved to see that Jimmy has seen her message. However, as they talk, Jimmy gets mad when Sarita asks him why has he not given up his ranch up to Enrique. She then says that she wants hostilities to end between Jimmy and Enrique. Meanwhile, Pancho hears a huge roar, and a prehistoric creature from the dawn of time, the Beast of Hollow Mountain, makes its first appearance (albeit offscreen), eating him alive.
Jimmy and Felipe begin to worry now that the Panchos have still not returned home. Felipe is stunned to find out that Jimmy says he will be leaving tomorrow and leaving Felipe in charge of the ranch. Panchito comes to their door, crying that his father has not returned. The three go into the swamp but only find his sombrero. They figure that quicksand was not the cause for Pancho's death. Panchito, grief-stricken, tries to go after his father in the swamp, but is stopped by Jimmy and Felipe. Meanwhile, a festival is going on in town, and women are busy gathering food and putting up streamers and displays. Jimmy talks to Don about Pancho's death. Don says that Panchito can be cared for in a foster home he has prepared for him. But grief-stricken Panchito is so sad he says he will not be friends with Jimmy again. Jimmy again meets Sarita and tells both her and Don that he will be moving himself and his cattle today, leaving the land for Enrique. He says goodbye to Sarita and then leaves town.
The festival is well underway, with dancers and firecrackers entertaining crowds. Enrique is delighted to find out that Jimmy is leaving and now devises his plan to stampede the cattle away from the station and make them his. The men decide to laze around for some time before stampeding the cattle and lie down and drink water together. Meanwhile, as Sarita prepares to wed Enrique, Panchito decides once and for all to go to the swamp after his father. Margarita, the assistant of Don Pedro, tries to stop him, but he gets away. Margarita tells Sarita about Panchito's departure and she goes out to stop him.
At the ranch, the Beast from Hollow Mountain appears and kills one of the cattle, forcing the others into a stampede. The cattle race toward the village where the festival is taking place. Jimmy and Felipe hear them coming and race to stop them. Enrique and his men become aware of the cattle as well, but their efforts to stop them are futile. The cattle stampede into the village, causing much panic and disrupting the festival. Don notices the stampede and blames it on Enrique, who in turn gets mad at one of his men whom he had told to stampede (but not toward the village). Just then, Margarita tells them that Jimmy and Sarita have gone after Panchito. Enrique and his men decide to follow Jimmy and Sarita who says she will not marry Enrique until this is all over.
While Panchito is in the swamp, searching for his dad, he is attacked by the Beast, which chases him across a river and to the small cottage, where Sarita greets him and the two decide to hide in the cottage. The Beast arrives at the cottage and manages to break in. Jimmy arrives and distracts it with his gun, causing it to lose interest in Panchito and Sarita. He orders them to get out of the cottage and while they flee to Panchito's horse, Jimmy continues to distract the Beast and leads it up a mountain. While he is dealing with the creature, Enrique comes back and attempts to kill Jimmy, but the sight of the Beast causes his horse to buck and throw him off. The Beast chases Enrique across a swamp and onto a plain, where Jimmy grabs him and the two flee on Jimmy's horse. They soon come to a steep slope, and are forced to slide down on their horse and are thrown off at the bottom. The Beast follows them down.
Jimmy and Enrique flee to a small cave with the Beast in pursuit. Sarita rounds up Don and other cowboys to come to Jimmy and Enrique's aid. The Beast manages to reach Enrique and pull him out of the cave. The Beast kills Enrique and then turns on Jimmy, who is only saved when Sarita and the other cowboys fire at the Beast and distract it. While the Beast chases them, Jimmy and Felipe head over to the tar pit. When the Beast arrives, Jimmy grabs a rope and whacks the Beast's nose with it. He throws the lasso around a tree branch, hoists himself upwards on the rope, and begins to swing back and forth, barely out of the Beast's reach. Taunted by this so-close stunt, the Beast walks forward a few steps and gets its feet caught in the tar. It roars helplessly as it begins to sink down into the tar while Jimmy is reunited with Sarita, Panchito, Felipe, and the others. Sarita weeps and the others, including Jimmy, look on sadly as the Beast, roaring in agony, dies in the black, sticky tar pit. They stare at the pit for a few seconds and then walk slowly toward their horses.Guy Madison as Jimmy Ryan
Patricia Medina as Sarita
Carlos Rivas as Felipe Sanchez
Eduardo Noriega as Enrique Rios
Julio Villarreal as Don Pedro
Mario Navarro as Panchito
Pascual García Peña as Pancho
Lupe Carriles as Margarita
The movie was filmed back-to-back in both English and Spanish at Churubusco Studios in Mexico City and released as La bestia de la montaña in Mexico.
Willis O'Brien's story that inspired the script for this film also inspired his earlier, unproduced script The Valley of the Mist, which would later be made as The Valley of Gwangi by O'Brien's protégé Ray Harryhausen.
While the bulk of the stop motion work was done with replacement animation the close ups of the dinosaur were filmed using an articulated puppet.
This was the first film with stop motion effects shot in widescreen and color.
On April 14, 2017, the film was "riffed" as Episode Five of the new Season Eleven of Mystery Science Theater 3000" on Netflix.