This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
馺 combines 馬 (horse) with 及, which likely contributes the sound. The exact historical development is uncertain.
A horse 馬 beside 及, but the meaning is unclear. Imagine a horse trying to catch up (及 suggests reaching or catching up), but treat this as a guess.
For ソウ, imagine a horse galloping so fast it creates a 'sou' sound: sou -> ソウ.