This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
駧 combines 馬 (horse) with 同, which likely contributes the sound. The exact historical development is uncertain.
A horse 馬 beside 同: perhaps a horse that runs together with others, but the meaning is unclear.
For トウ, imagine a horse running through a tall tunnel: tunnel -> トウ, and the horse 馬 races through.