Albert Barnes Commentary Psalms 135:8

"Who smote the first-born of Egypt, Both of man and beast;" — Psalms 135:8 (ASV)

Who smote the firstborn of Egypt - As the last and the greatest of the plagues brought upon the Egyptians; the chief and crowning judgment that compelled them to let the children of Israel go, and which was in fact the judgment that secured their freedom. This is evidently selected here for this reason, instead of recounting all the plagues brought upon the Egyptians.

Both of man and beast - (Exodus 11:5). The marginal note, as in Hebrew, reads: From man to beast. This means including both; striking both.