mother, mare
媽 centers on a female parent or a female horse: mother and mare. Both meanings share the idea of a female figure, with mare being a specific animal application.
媽 combines 女 (woman) with 馬 (horse), which likely contributes the sound. The character is used for mother or mare, but the exact historical development is uncertain.
A woman 女 beside a horse 馬: picture a mother mare with her foal, or a mother caring for her child like a mare with her young.
For ボ, imagine a mother horse bowing her head to nuzzle her foal: bow -> ボ, and the mother mare shows affection.