What is rare is finding a Union hero in a Western.
Oh God. Here are a few off the top of my head:
Wyatt Earp in all kinds of Westerns. He tried to join a Union regiment but was too young. Virgil did serve in an Illinois regiment.
Wild Bill Hickock in many Westerns.
Linus and Zeb Rawlings, How the West Was Won
Nathan Brittles, She Wore a Yellow Ribbon
Kirby Yorke, Fort Apache
and Rio Grande
Seth Adams, Wagon Train
Paladin, Have Gun Will Travel
Lucas McCain, The Rifleman
Cord McNally, Rio Lobo
John Dunbar, Dances With Wolves
Colonel Thomas, The Undefeated
That's because the classic Western is about a racial mythology.
So you claimed way back up thread and were given plenty of contrary examples.