Back in the mid-1950s, if you wanted to make a giant monster you worked with a basic toolkit: Stock footage, dim lighting, and editing tricks. And yet somehow this mega-spider from 1955's Tarantula is more convincing than today's CGI mega-whatevers.
Maybe it's just because tarantulas move in such a creepy way that they can't help but seem scary when enlarged and unleashed on cows. Or maybe it's been scientifically proven that tarantulas fill our brains with fear.
If you like Syfy Movies of the Week, you need to check out Tarantula to see how it was done old school.
Tarantula via IMDB