name of a legendary swift horse
駬 is a rare kanji that names a legendary swift horse. Its meaning is specific to that mythical creature and does not extend to general horse or speed terms.
駬 combines 馬 (horse) with 耳 (ear), likely as a phono-semantic compound where 耳 contributes the sound and 馬 indicates the meaning relates to a horse. The exact historical origin of the legendary horse name is uncertain.
A horse (馬) with distinctive ears (耳) that belongs to legend: 駬 is the name of that swift mythical horse.
For ジ, imagine the legendary horse galloping so fast it leaves a G-shaped trail: G -> ジ, and the horse's ears stream back in the wind.