Up until 1990, it was beef tallow. After that, it was vegetable oil, due to political pressure of course.
Some restaurants have re-introduced beef tallow
https://www.thedailymeal.com/1833047/chain-restaurants-that-use-beef-tallow/
Yep, i remember when they switched- hated the fries from that pount forward. We make our own now with duck fat. We cut lert over boiled potatoes into smallish cubes and fry those in a layer of duck fat too- life doesnt get much better. (Well, it could get better IF McDonalds went back t9 beef tallow!)