What word would someone around the era of the Wild West (1850) use to describe a "posh fancy-pants"? I see that "fancy-pants" first known usage was in the twentieth century, so it's a no-go. Now, the sociolect of the person saying this would be that of a coal miner in Texas, which is quite specific, but should give a general idea of their speech.
"Tinhorn" and "City Slicker" are possibilities.
“Tinhorn” (used in the 19th Century) referred to an unscrupulous, unskilled, self-important or low-class gambler. It could also refer to a man pretending to have money or influence, or someone who was flashy; a dude or a phony. The term originated from a game where three dice were rolled down a chute onto a flat area. True West Magazine truewestmagazine.com/what-does-the-word-tinhorn-mean
City slicker (probably not used until the early 20th Century) is an idiomatic expression for someone accustomed to a city or urban lifestyle and unsuited to life in the country. The term was typically used as a term of derision by rural Americans who regarded them with amusement. It may refer to a "fop", or it may be a derogatory term for a person wearing inappropriate city-type business attire, particularly with a brusque or condescending attitude in areas where local residents are offended by an arrogant attitude combined with disdain and lack of respect for rural people and lifestyles. Wikipedia
High Hatters. My kin have always called them high hatters. From Great Gran Paw to Paw Paw, down to my Paw....Them there fellers are high hatters, come down here from Birmingham. Hey, don't knock it, that's how them old timers spoke when you could get them to speak. The "fancy pants" you speak of were labelled high hatters due to their derby style hats would sit higher upon their heads than a ropers hat or a ten gallon hat would.