Musar su Salmi 121:5

He then cited, “These are the sons of Tziv’on, Ayah and Anah — the same Anah who found the mules (yeimim) in the desert.” “The word yeimim is written missing a yud (ימם).” The concept here is that there is a certain kind of destructive spirit, Heaven spare us, that focuses in particular upon those guilty of immoral acts from which mamzeirim are produced (i.e., adulterous or incestuous relations bearing the penalty of excision). These spirits are called yeimim. It is written without a yud to indicate that they were created on the eve of the first Shabbos just at dusk (relating yeimim to yom — day; it is plural but missing the plural yud to indicate that it is doubtful to which day this creature belongs). The proper place of these destructive spirits is upon the high mountains, where no seed or tree can be planted. That barren region is their proper place and these are the strange creatures that Rabbi Yitzchak saw. Anah, mentioned in the verse above, once visited that mountain where these creatures adhered to him. Therefore, as the Sages tell us in Pesachim (54a), he himself gave birth to mamzeirim and that he also was the first one to graft different species and produce the mule (a hybrid of a horse and a donkey). The Zohar mentions in the continuation of that passage that all desolate mountains are their dwelling place, however, concerning those who engage in study while they travel it is written, “Hashem your shelter is at your right hand” (Tehillim 121:5). Nevertheless, a person should avoid places where there is no human habitation.