The original manuscript has the plural beasts (written with a capital B), but Oliver Cowdery changed this plural to the singular when he copied the text into the printer’s manuscript. This loss of the plural s may be related to the same kind of change that took place in verse 9, where plans was replaced by plan when Oliver copied the text from 𝓞 into 𝓟. The question here, as with verse 9, is whether Oliver accidentally added the plural s in the original manuscript or whether the plural was actually intended.
Preceding beast(s), we have a singular man (“a sacrifice of man”); and following beast(s), we have a singular fowl (“any manner of fowl”). So one could argue that all three nouns in the list should be singulars. There are only two other examples in the Book of Mormon involving a conjunction of beast(s) and fowl(s)—and neither one has any mixture of singular and plural within the same conjunctive structure:
But in the last example the singular conjunctive noun phrase “whatsoever beast or animal or fowl” is conjoined to a preceding plural conjunctive noun phrase, “their flocks and herds”. Thus this last example could be viewed as a case of mixture in number across a larger and more complex conjunctive structure.
In the King James Bible, there are numerous passages where beast(s) and fowl(s) collocate. In most cases, there is agreement in number, either both singular (16 times) or both plural (29 times). Nonetheless, there are four passages in the King James Bible where such a collocation disagrees in number:
So these mixtures from the English of the King James translation support the possibility of allowing the plural beasts and the singular fowl to be conjoined in Alma 34:10. Interestingly, these four cases of mixture in the King James Bible are all instances of singular beast and singular fowl in the Hebrew original, which one could take as support for the current reading in Alma 34:10. But since the mixture is possible in English, the critical text will here follow the reading of the earliest textual source, the plural beasts and the singular fowl (both are extant in 𝓞).
Summary: Restore in Alma 34:10 the plural beasts in “neither of beasts neither of any manner of fowl”), the reading in 𝓞; there is nothing wrong in English with conjoining the plural beasts with the singular fowl in this passage (such conjunctive phrases are also found in the King James Bible).