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.
A word for “rabbithole” appearing in the Declension of Nouns from the early 1930s, a combination of ᴹQ. lopo “rabbit” and ᴹQ. tundo “hole”. In The Etymologies of the 1937, ᴹQ. tundo became “hill”, but the word lopotundo might still be used for the entrance to a rabbit warren, referring to the mound at the entrance rather than the hole itself.