In the US and Canada, a wild or partially tamed pony or mustang of the western plains (6)

I believe the answer is:

bronco

(Other definitions for bronco that I've seen before include "Wild horse of W. US" , "Lively horse" , "American wild horse" , "Wild horse of the western US" , "Wild horse of the West" .)

I've seen this clue in The Independent.

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