Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is an old-fashioned western, for 1971 (at least when compared to all of the great Revisionist Westerns and Spaghetti Westerns that were being made in that era) with a good story, based on a book by Louis L'amour. It has some comic moments and plenty of western cliches including Indians and bad guys who can't shoot guns straight and die by the dozens while the good guys sit in the open untouched, and more. Yul Brynner plays Catlow, a charming outlaw and old War buddy of Richard Crenna, who is a US Marshall chasing him with an outstanding warrant, while Leonard Nimoy is a hired bad buy who is chasing him for his boss and for his own revenge, and everybody is being chased and attacked by the Indians.
At one point in the movie, Brynner is riding at the front of his gang in a dry wash when we hear rattling and see a close-up shot of a rattlesnake. His horse is scared and rears up and somebody immediately shoots the rattlesnake (we assume it's Brynner, but we don't see him pull his gun because his horse is in our way) then we see the snake bounce into the air in a cloud of dust. (The typical killing a rattlesnake movie shot.) A split second later we see a bullet hole shot through Brynner's straw hat. He says "Damned rattler shot back!" But he soon discovers it was really a missed rifle shot from Leonard Nimoy who fired on him from up on the rim of the wash.
The first snake we see is a ridiculous prop with a bad paint job and an elevated rattle that somebody is shaking. It turns out it's the same terrible snake used in the movie The Last Challenge made a few years earlier in 1967. The production company for both movies was MGM, so they probably borrowed it from The Last Challenge because there are no rattlesnakes in Spain where Catlow was filmed. Or maybe it's their standard pre-killed rattlesnake shot and I can look forward to seeing it in other MGM westerns. The next snake we see is a real snake, either dead or alive, getting blown to pieces. It looks nothing like the first snake, and the entire sequence looks like it was very obviously taken from another film. But not from The Last Challenge, at least. Whey they did not show this snake as the before-shooting rattlesnake, is a mystery.