Thanks to the movies and mid-20^th century TV, when we think of men’s underwear in the Old West, we usually picture the union suit, a garment that looked like a cross between a onesie and today’s longjohns, with buttons up the front and a flap in the back to accommodate nature’s needs.  But union suits weren’t always a single piece — it was possible to buy shirts and pants separately.  They also came in an above-the-knee length (for the especially hardy).

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Sears & Roebuck catalogue

Less well-known were garments called “linens”, from the material they were made out of.  Linens could be long, though they were usually knee-length or shorter.  They had buttons or grommets with laces, and some had wide waistbands.  Even more obscure was a cotton garment with buttons and ribbed legs that’s quite similar to the recently-popularized “boxer-briefs”.  From my research, I believe these were among the first to be considered “drawers”.