I've encountered the horse defense both in verbal conversations and even in magazine articles. It's interesting that modern TKD spends so
much time developing high kicks for kicking soldiers off horses. Without
a springboard or a nearby building, such a feat seems improbable.
If any of you are interested in reading about the RECENT development of
high kicks in Hapkido, check out: Hapkido: Traditions, Philosophy, Technique by Marc Tedeschi . The book contains numerous interviews with prominent Hapkido masters along with statements that they invented particular high/spinning kicks.
Viking answer: Lance the horse, wait for it to fall, decapitate the rider.