The noun ᴱQ. yaksi “cow” appeared in the Qenya Lexicon of the 1910s as a derivative of the early root ᴱ√Ẏak “a head of cattle” (QL/105). The contemporaneous Gnomish Lexicon also had ᴱQ. yaksi, but this form was deleted and ᴱQ. yakse was given as the cognate of G. gach “milch cow” (GL/36).
Conceptual Development: In Early Qenya Word-lists of the 1920s, the word for “cow” was ᴱQ. mui (PE16/132), likely related to G. mûs “cow” from the Gnomish Lexicon of the 1910s which had a deleted variant {mui} (GL/58). This probable relationship was suggested by Patrick Wynne and Christopher Gilson (PE16/132).
Neo-Quenya: For purposes of Neo-Quenya, I would adapt this word as ᴺQ. yaxë from the Neo-Root ᴺ√GYAK; Helge Fauskanger used this word for “cow” in his NQNT (NQNT).
A word for “cow” in the Gnomish Lexicon of the 1910s with deleted variant {mui}, a feminine form of G. mû “ox” (GL/58).