Early Quenya
yaksi
noun. cow
yakse
noun. cow
mui
noun. cow
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
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).