strong horse
駜 centers on a horse that is robust and powerful: a strong horse. The meaning is specific to equine strength.
駜 combines 馬 (horse) with 必, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
A horse 馬 that must (必) be strong: imagine a powerful steed that you can always rely on.
For ヒツ, picture a strong horse that hits (ヒツ) the ground hard with its hooves, showing its power.