lift up, err, hopping
騫 centers on a horse's unsteady, upward motion: lifting up, hopping, or making an erratic leap. The idea of error comes from a misstep or deviation in that motion.
騫 combines 馬 (horse) with a phonetic component that also suggests lifting or rising. The character likely originated to describe a horse rearing or leaping upward.
The top looks like a roof or cover being pushed upward, and the bottom is 馬 (horse). Picture a horse hopping and lifting its front legs, nearly hitting the stable roof.
For ケン, imagine Ken the cowboy lifting a hopping horse: Ken -> ケン, and he lifts the horse up when it errs.