20 Million Miles to Earth (1957)

In this typical '50's sci-fi, a space ship returning from a secret mission to Venus crashes off the cost of Sicily with only one survivor: Col. Calder (William Hopper, lethargic middle-aged leading man). A little kid finds a cylinder containing an embryonic creature called the "Ymir," which grows by leaps and bounds into a half-man, half-reptile looking like an oversized Creature from the Black Lagoon. It gets loose in Rome and ends up,in the Coloseum, after a fight with a bull elephant from the zoo. The plot and acting are ridiculous. The only saving grace is the special effects by the immortal Ray Harryhausen, which you'll recognize immediately.