I looked it up in the 60 pound dictionary. It's a tribe in Washington State, now gathered somewhere (eyes are bad) in Oregon, or an Indian pony. We always used it to mean a wild horse and we put an adjective in front of it: nasty cayuse, unbreakable cayuse, etc. Worst one we ever had was named Hemi. That's short for hemmrrohoids. He'd unload you on any pile of nail filled lumber he could find.
I hope that little boy grows up healthy.