וַיִּרְדְּפ֤וּ מִצְרַ֙יִם֙ וַיָּבֹ֣אוּ אַחֲרֵיהֶ֔ם כֹּ֚ל ס֣וּס פַּרְעֹ֔ה רִכְבּ֖וֹ וּפָרָשָׁ֑יו אֶל־תּ֖וֹךְ הַיָּֽם׃
애굽 사람들과 바로의 말들, 병거들과 그 마병들이 다 그 뒤를 쫓아 바다 가운데로 들어 오는지라
Rashi
כל סוס פרעה (The word סוס is singular) — But was there only one horse? But the use of the singular teaches us that they were all accounted before the Omnipresent as only one horse (cf. Mekhilta).
Chizkuni
ויבאו אחריהם, “they followed after them;” They might not have done so knowingly, but seeing that everything in front of them was darkness, they did not realise that they had done so until they were into what had been sea moments before.
Or HaChaim
וירדפו מצרים, Egypt pursued, etc. Even though the Torah says: "the cloud remained in place" (verse 20), what is mentioned here occurred in daylight already at a time when the cloud was not as impenetrable so that the Egyptians could begin to discern where the Israelites were positioned. This enabled them to start their pursuit.