Isaiah 11:8 (King James Bible) and the weaned child shall put his hand on the cockatrice’ den
The Book of Mormon text has the standard possessive form cockatrice’s rather than the King James cockatrice’, not only here in 2 Nephi 21:8 but also later when this same Isaiah passage is quoted once more near the end of 2 Nephi:
Although in this second instance, Oliver Cowdery initially wrote cockatrice in 𝓟 (spelled as cocketrice), he immediately corrected the word by adding the possessive s. In fact, he made sure that the s was placed so that it continued as an extension of the offstroke from the originally final e of cocketrice. This kind of connecting correction is characteristic of Oliver’s corrections elsewhere in the manuscripts when he accidentally omitted a final s and then immediately added it. In addition, the ink flow for the s is unchanged. And finally, the s is rather small in size, as if the s was expected. In other words, Oliver did not insert a highly visible s, as if he were editing the word. Oliver seems to have just written the base word cockatrice (spelled cocketrice) and then added the s as a matter of course. Thus in 2 Nephi 30:14 as well as in 2 Nephi 21:8, the Book of Mormon text supports the use of the standard possessive form cockatrice’s.
In Early Modern English, nouns and verbs ending in an /s/ in an unstressed syllable often lacked the expected -(e)s inflectional ending in contexts where normally it would have occurred, such as the plural or possessive for nouns or the third person singular present for verbs. Charles Barber (Early Modern English [Edinburgh: Edinburgh University Press, 1997], page 145) gives the example from Shakespeare of “poor Clarence’ death” (Richard III, act 1, scene 4, line 186); in modern English we would expect “poor Clarence’s death”. In the same way, the possessive in the King James text for Isaiah 11:8 reads cockatrice’ rather than cockatrice’s (the modern English form for the possessive).
The earliest sources in the Book of Mormon show the modern English form cockatrice’s, although in the manuscripts the apostrophe was not originally written (thus cockatrices here in 2 Nephi 21:8 and cocketrices in 2 Nephi 30:14). In the first instance, the apostrophe was added in 𝓟 itself by the 1830 compositor (who used heavy black ink to mark the accidentals for this part of 𝓟). The apostrophe was not marked in 𝓟 in the second instance, but the compositor added it when he set the type.
The critical text will continue to follow the modern form cockatrice’s in both these instances. Of course, the extra s might have been added as Joseph Smith read off these two passages or as Oliver Cowdery wrote them down in 𝓞 or copied them into 𝓟. Nonetheless, the modern form cockatrice’s works and it will therefore be retained in the critical text. For another example of such a minor grammatical adjustment in the Isaiah quotations, see the discussion under 2 Nephi 13:7 regarding the use of the indefinite article a(n) before h-initial words. For a case in the Isaiah quotations that shows variation in a word form, see the discussion under 1 Nephi 21:23 regarding the word toward(s).
Summary: Maintain the modern possessive form cockatrice’s in 2 Nephi 21:8 and 2 Nephi 30:14; the King James Bible has cockatrice’, an expected possessive form in Early Modern English.