Charles Ellicott Commentary Exodus 9:20-21

"He that feared the word of Jehovah among the servants of Pharaoh made his servants and his cattle flee into the houses. And he that regarded not the word of Jehovah left his servants and his cattle in the field." — Exodus 9:20-21 (ASV)

He that feared ... — Some impression, we see, had been made by the preceding plagues, and the warning was taken to some extent; but it was otherwise with many. So in Gospel times, Some believed the things which were spoken, and some believed not (Acts 28:24). The result was death, both to the cattle and their keepers (Exodus 9:19). .