As far as I know, all the peoples of the ancient times that could be called "rider peoples" used to dress in trousers. This obviously englobes celts, all the germanic hordes, and even the asiatic peoples from middle east to China. Being romans and greek armies mainly based in a powerful and disciplinated infantry, they didn't give really much attention to cavalry. In Rome cavalry was just reserved to those who could afford it. That's why since the times of the roman republic they used to contract as mercenaries many cavalry troops from celt, numidian or germanic origins, and always as some kind of "auxiliary forces", not as the main body of their armies.

Even with this, if you can observe sculptures showing legionaries, you will notice that roman soldiers used to wear short pants (surely leather pants) up to thier knees, under the short "skirts" , or the bottom of their "tunicae" that was part of their uniform.

Here u can find some modern views of it:

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In my opinion, the use of those pants was reserved for war duty, and they were avoided in formal occasions. But one thing is clear,... its use was more spreaded as far as romans were more mixed with the "barbaric" peoples after the creation of the empire and the increasing importance of cavalry on their armies.

¡Salud!

T O N O