This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
駫 combines 馬 (horse) with 光 (light), but the exact historical development and original meaning are uncertain. It may have been a phono-semantic compound, but the connection is unclear.
A horse (馬) glowing with light (光) might suggest a spirited, strong horse, but the actual meaning is uncertain.
For ケイ, imagine a horse named Kay who shines with light: Kay -> ケイ, and the horse (馬) is bright (光).