fast horse
騏 centers on a swift, excellent horse. The meaning is narrow and concrete: a fast horse.
騏 combines 馬 (horse) with 其, which likely contributes the sound キ and may have originally suggested a fine or patterned horse.
A horse 馬 beside 其, which looks like a basket or winnowing device. Imagine a horse so fast it can outrun the wind from a winnowing basket.
For キ, picture a key (キー) that unlocks the stable of a legendary fast horse: key -> キ, and the horse bursts out at full speed.
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(Royal) light cavalry