-Oracle Bone Inscriptions
The reason why we pronounce "horse" is because the word is a foreign word.
As we know, horses were first domesticated in the grasslands of Central Asia and West Asia 5,000 years ago, where the Indo-European pronunciation was "aspa" and the vowel sound was changed to sapa, that is, "Sima" and "Yun Qi" in Chinese, and they were introduced to China in Xia and Shang Dynasties. Because Chinese is mainly monosyllabic, the last sound "ma" has been missed.