Early Noldorin
bregil
noun. mare
brois
noun. mare
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 words ᴱN. bregil “mare” appeared in Early Noldorin Word-lists along with a variant brois, both feminine forms of ᴱN. brog “horse” (PE13/139). In the Gnomish Lexicon of the 1910s, there was a different list of “mare” words: G. bros, bross and broch, though still based on G. brog; the earliest version of the entry had only the form bross (GL/24).
Neo-Sindarin: Since -il continues to appear in later Sindarin as a feminine suffix, I would adapt this word as ᴺS. regil “mare” based on the later root √ROK used for “horse” words from the 1930s and forward; compare for example [N.] brennil “lady” versus [N.] brannon “lord”, both based on brann.