Well... A century ago, Colorado imported 50 elk from Wyoming to re-establish dwindling herds. The elk were transported and released in Idaho Springs and in the Greenhorn Mountains in Pueblo County

CPW conducted the first sheep transplants in the 1940s, including planting bighorns between Georgetown and Silver Plume.

And so on.

The Mustangs however? Not transplants. Roamers perhaps-but not imported. 
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The Great Escape Mustang Sanctuary: Boulder CO 80304
67,000 ESTIMATED WILD HORSES & BURROS ON THE RANGE
https://greatescapesanctuary.org/ranchsanctuary/
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Mustang Wars: Saving Colorado's Wild Horses - Aspen Peak Magazine
https://aspenpeak-magazine.com/saving-colorado-wild-horses