Ether Horse
Just like horses were disseminated to new continents and worlds by early explorers on the Prime, giant sea horses were brought to the Ethereal long ago to serve as mounts. See, giant sea horses were used to a life spent in the sea and had traditionally served undersea races as mounts--it wasn't too much of a stretch for canny bashers to bring them onto the Waveless Sea.
Many sea horses eventually escaped in sufficient numbers to start wild sea horse herds on the Etheral. Many died off in the absence of their traditional nutritional sources; however, the rest lived by gleaning nutrition directly from the ethereal medium, eventually evolving into purely ethereal creatures. The resulting wild herds are now known as ether horses.
Wild ether horses are encountered in the unfathomed Deep and periodically caught and tamed to the saddle. Moreover, it's said that the Etherfarer Society keeps a stable of these creatures domesticated for use by the high-ups within their groups. Clueless travelers trespassing in Etherfarer territory encounter a cavalry of high-ups, mounted on swift, armored ether horses and armed with lance and sword.
Ether Horse: AC (7, or 5 in armor); MV Sw 21; HD 4; THAC0 17; #AT 1; Dmg 2d4 (bite); SA serves as mount, specially armored ether horses serve as war mounts; SZ L (10' long); ML average (10); Int low (2); Al N; XP 120.
Originally appeared in A Guide to the Ethereal Plane (1998).