The Ship of Monsters (1960)
Sexy space women in search of virile Earth mates land their rocket ship in the middle of a Mexican singing Western. If they don't get the volunteers they want, they'll unleash a quartet of monsters capable of devouring all terrestrial life. Unfortunately, their first "recruit" -- a boastful cowhand with a penchant for verse and falling off his dancing horse -- turns them down. Guess who Earth is now depending on to save it?
The space babes look good (females always pilot spacecraft in evening dress, don't they?), and so do the monsters: an evil brain, a spider-thing, a cyclops, and a talking saber-toothed skeleton. The sci-fi gear is cool in a 1950s sort of way, sort of Rocketship X-M meets Things to Come.
All you Atomic Age Cthulhu and Astounding Adventures fans take note!