The normal Quenya word for “tooth”, appearing in The Etymologies written around 1937 derived from the root ᴹ√NELEK of the same meaning (Ety/NÉL-EK), where the final k became t. Its plural form nelki [nelci] indicates a stem form of nelk- [nelc-], since in most inflected forms the word would undergo the Quenya syncope and loose the second e. A variant form nelke (EtyAC/NÉL-EK) may reflect a reformation to align with those inflected forms.
Conceptual Development: In the Qenya Lexicon of the 1910s Tolkien had ᴱQ. nele (neli-) “tooth” under the early root ᴱ√NELE (QL/65), a word also mentioned in the contemporaneous Gnomish Lexicon (PME/65). In Early Qenya Word-lists of the 1920s Tolkien instead had ᴱQ. nyat “tooth” (PE16/136), but that seems to have been a transient idea. ᴹQ. nelet first appeared in Common Eldarin: Noun Structure from 1936 alongside variant nelke, but it was replaced by nele and then later nelke was deleted as well (PE21/56 and note #12). Both nelet and variant nelke were restored in The Etymologies (see above).
Primitive √nelek “tooth” was mentioned in Common Eldarin: Noun Structure from the early 1950s, but its Quenya derivatives were not listed.
Neo-Quenya: For purposes of Neo-Quenya, I’d use nelet (nelc-) “tooth” as the traditional form of this word, with nelcë as a modern variant with more regular inflections.
A noun in The Etymologies of the 1930s glossed “crow” derived from the root ᴹ√KORKA of the same meaning, replacing rejected ᴹQ. karko derived from ᴹ√KARKA (Ety/KARKA).
Conceptual Development: The Qenya Lexicon of the 1910s had ᴱQ. karon “crow” (QL/45), a word that was also mentioned in the contemporaneous Poetic and Mythological Words of Eldarissa (PME/45). In the Quendi and Eldar essay from 1959-60, Tolkien had Q. quáco “crow” derived from primitive ✶k(a)wāk (WJ/395), but in notes from the late 1960s Tolkien instead had Q. {koake >>} quácë “frog” < ✶kāwāk, with primitive ✶kā for “crow” (VT47/36).
Neo-Quenya: Since quácë “frog” is later than (and possibly replaces) Q. quáco “crow”, I prefer corco as “crow” for purposes of Neo-Quenya.