St Jerome has in Gen 7:23, “ab homine usque ad pecus” but pecus is nominative (or perhaps genitive if the word is pecu) and not accusative. Am I misunderstanding something here?
The complete verse is
Et delevit omnem substantiam quae erat super terram, ab homine usque ad pecus, tam reptile quam volucres caeli: et deleta sunt de terra. Remansit autem solus Noe, et qui cum eo erant in arca.
There are two main words with nominative form pecus:
with related meaning. The latter is for an individual member of a herd, the former means herd or, generically, cattle.
The KJV translation is
And every living substance was destroyed which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark.
The Lewis and Short entry for pĕcus, pecŏris states
In late and eccl. lat. the distinction between pecus, f., and pecus, n., nearly disappears, and the latter is found in all senses of the words; cf. Vulg. Lev. 20, 15; id. 2 Par. 14, 15; id. Isa. 66, 3.
So the neuter form is what's meant here and the accusative is the same as the nominative in all neuter nouns.
pecus is actually a neuter noun, so its nominative and accusative singular look the same! (And ad takes an accusative.)
Full morphology of this word here.