When health concerns developed over the use of saturated animal oils in french fries, the fast food industry changed its cooking oil. They switched to a polyunsaturated vegetable oil. That was good. But in treating that unsaturated oil to make it better for frying, they ended up with trans fats. That was bad.
Trans fats aren't as rampant as they were a few years ago, but their ability to accumulate over a lifetime still makes them dangerous.
The term "saturated" doesn't refer to how densely fatty something is. It refers to the amount of hydrogen in a fat molecule.