The word ᴹQ. yat (yak-) “goat” appeared in the Declension of Nouns from the early 1930s (PE21/24).

Conceptual Development: In the Qenya Lexicon of the 1910s and Early Qenya Word-lists of the 1920s, ᴱQ. yakko was “ox” (QL/105; PE16/132). In Common Eldarin: Noun Structure of the early 1950s, Tolkien gave the possibly-related primitive form ✶najak- for “goat”, with masculine and feminine variants ✶najakō “he-goat” and ✶naikē “she-goat” (PE21/82).

Neo-Quenya: Helge Fauskanger used ᴺQ. naico for (male?) “goat” in his Neo-Quenya New Testament (NQNT) as a derivative of ✶najakō, and I would recommend this over ᴹQ. yat for this purpose. However, I’d recommend ᴺQ. nyéni for a female goat (adapted from ᴱQ. nyéni) since it (a) is more distinctive than *naice and (b) does not conflict with ᴹQ. naike “sharp pain”; Helge Fauskanger used nyéni in NQNT as well.

A noun for “ox” appearing in the Qenya Lexicon of the 1910s with variants yakko and yako, derived from the early root ᴱ√Ẏak “a head of cattle” (QL/105). In the contemporaneous Poetic and Mythological Words of Eldarissa Tolkien had yakk- “ox” (PME/105). In Early Qenya Word-lists of the 1920s, the word for “ox” was also yakko (PE16/132).