I think the term rat rod came from the old Ed Roth Rat Fink cartoons and designs that he drew in the 50's and 60's? The only source for new or replacement parts was the junkyard in that era. There were very few muscle cars to customize, you had to chop and channel a 50 merc to make it look bad and you had to do it yourself!

Today, I think the rat rod culture (if it can be called that) is great in that it keeps those old rust buckets on the road that would likely be crushed otherwise. There is a lot of ingenuity and creative design that goes into those cars. I think the faux rust and worn paint is silly because rust isn't very hard to find in old car parts but for the most part those rat rod mods a cool.