Qenya
rokko
noun. (swift) horse
olombo
noun. horse
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.
A word for horse in The Etymologies of the 1930s, derived from the root ᴹ√LOP (EtyAC/LOP). This root did not appear in The Etymologies as published in The Lost Road, but Carl Hostetter and Patrick Wynne reported it in their Addenda and Corrigenda to the Etymologies (VT45/28). Tolkien first gave the root as ᴹ√LOB, and it seems the form olombo was derived from this earlier form, and was not updated after {ᴹ√LOB >>} ᴹ√LOP, as pointed out by Hostetter and Wynne.
Conceptual Development: The word ᴱQ. lópa “steed, horse” appeared in the Qenya Lexicon of the 1910s as a derivative of the early root ᴱ√LOPO that was the basis for “horse” words in the Qenya Lexicon (QL/56).
Neo-Quenya: Given the dubious derivation of olombo, I recommend limiting yourself to better attested Q. rocco “horse” for purposes of Neo-Quenya. If you do use it, it should probably be revised to ✱olompo.