The Madmen Of Mandoras (1963)

Starring Walter Stocker, Audrey Caire, Carlos Rivas, John Holland, Marshall Reed
Directed by David Bradley
(actor & director credits courtesy IMDB.com)

Enemy agents kidnap a professor who's invented an antidote to a deadly gas, and take him to a foreign nation where one of history's greatest villains may still be alive. 

The main point of interest in this movie is the identity of the supposed man behind the plot, which I won't reveal here, but will say the film was later re-released with added scenes and a new title that gave his identity away.  The scenes without that villain and the sci-fi premise behind his preservation are pretty routine, and a largely unknown cast and an uninspired screenplay don't help matters.  It was good to see familiar 1950s character actors Rivas and Nestor Paiva, and the premise is a good one, but would have made a much better movie in more skilled hands.  It's still worth checking out if you've never seen it, but for me, it just wasn't a highly entertaining film.

The Beast Of Hollow Mountain (1956)

Starring Guy Madison, Patricia Medina, Carlos Rivas, Mario Navarro, Pascual Garcia Pena
Directed by Edward Nassour & Ismael Rodriguez
(actor & director credits courtesy IMDB.com)

A cattle ranch owner in Mexico suspects a rival rancher when some of his cattle go missing, but a prehistoric creature is the real menace responsible.

A conventional western for the most part before revealing its sci-fi twist in the form of a stop-motion animated dinosaur, I found this film to be pretty entertaining, well-abetted by a cast of skilled Mexican character actors, who build audience empathy with their plights.  American import Madison isn't given too much to do, but looks good on a horse and brings the requisite toughness to a knock-down drag out fight.  As for the dinosaur animation, it's pretty well-done, despite movement by the beast's tongue that borders on the silly.  Don't expect anything too profound, and you'll have a good time watching this.

The Black Scorpion (1957)

Starring Richard Denning, Mara Corday, Carlos Rivas, Mario Navarro, Carlos Muzquiz

Directed by Edward Ludwig
(actor & director credits courtesy IMDB.com)

A pair of geologists in Mexico trace savage killings in the wake of a volcanic eruption and earthquake to a horde of giant scorpions, freed from their subterranean lair by the disaster.

Another of the giant insect movies of the 1950s, this one boasts excellent stop-motion animation effects overseen by Willis O'Brien, who pioneered creature effects of this kind in the classic King Kong.  This film doesn't compare to that masterpiece, but remains a fun effort, much in the same vein as 1954's Them!, with Denning and Rivas tracing the beasts back to their underground nest and attempting to re-trap them inside.  Although O'Brien and his team supplement the animation with effectively grotesque mockups of the scorpions' heads, these aren't integrated too well in the film, and according to Wikipedia, a number of scenes where we only see the scorpions' silhouettes represent unfinished work by O'Brien's unit because they ran out of money.  Nevertheless, I've found this to be one of the more enjoyable monster pictures of the 1950s, and one well worth revisiting to experience and appreciate O'Brien's wizardry.