Kurt R. A. Giambastiani’s “Fallen Cloud” novels are an alt-history series in which- at the beginning of the series- the western expansion of the settlers slowed dramatically when they ran into Native Americans who rode raptors. Pretty cool series.What about alt-history? An old west where racism doesn't exist? The problem with old west is going to be who will be the antagonist which is generally going to mean taking on one human group based on history against another. Unless you do a cowboys vs. aliens style game.
First Nations tribes riding dinosaurs a bit too far? Cut a couple million years off of that and they could be using giant flightless birds instead.
Or lean into it in the other direction- the First Nations tribes aren’t human, they’re the humanoid descendants of dinosaurs. Your own version of “Sleestacks”, if you will.
Then again, ripping off Julian May’s Saga of Pliocene Exile does introduce aliens into the (obviously) deep past. That background could change the nature of the American Westward Expansion.
FWIW, I had a fantasy campaign setting in which crashlanded Grays used their advanced technology to disguise themselves as Elves of legend. “Underhill” was their buried, crashed ship; temporal and spatial distortions of legend therin resulting from their use of stasis fields and multidimensional engineering. A similar approach could account for assorted Native legends. And they might have an interest in who controls the land…