The ancient Romans, as far as I know, did not wear pants for most of their history. However that seems like it would be very uncomfortable without them. Did they have any kind of riding chaps or trousers?
They wore braccae, kind of non-stretchy legging things which were of varying lengths. I've read they were first seen on the Gauls, not sure about that as definitely worn across iron age northern Europe. Iirc they started in auxiliary cavalry units.
Here is a nice article about the evolution of pants on cavalry units