媽 is a sound-and-meaning character, not a literal picture of a mother beside a horse. 女 supplies the meaning clue, while 馬 (mǎ) supplied the historical sound clue. Simplified 妈 keeps that same structure with 马 on the right. You already know both building blocks, so this very familiar family word also teaches an important reading habit: one side can hint at meaning while the other hints at sound.
A piece of culture
Family words echo Mandarin often doubles a syllable in familiar family words: 妈妈 is “mom”, just as 爸爸 is “dad”, 哥哥 is “older brother” and 姐姐 is “older sister”. Cantonese also uses 媽 and 媽媽, although the exact choice and pronunciation can vary by family and region. The repetition makes these among the first warm, rhythmic words many children say.