I won't bore you with my nightmare, which left me with a feeling of foreboding that I couldn't shake off for a while. But it made me curious about the origin of the word mare.
It has nothing to do with horses, as far as we know, but comes from a Germanic word mara, 'goblin, evil spirit'; something that sits on a sleeper's chest and makes them feel crushed or stifled. The same word appears as the second element in modern French cauchemar, the cauche- part being from cauchier, 'crush or trample', so I suppose it originally meant something like 'crush-goblin'.
Mara has close cognates in many Germanic languages and also in several Slavonic ones, as well as Old Irish morrigain, 'queen if the elves', 'demon of the corpses'., 'phantom queen', a mythological figure worth several blog posts on her (or perhaps their) own.
Mara may come from Proto-Indo-European (PIE) root *mer-, to do with something being worn out or damaged or more broadly mortal. If so, nightmare is related to a whole series of words, such as ambrosia, mortar (as in 'pestle and'), mortuary, murder, murrain, remorse, etc.
Interestingly Ancient Greek εφιάλτης (efialtis) has completely different linguistic roots from nightmare, but it meant a kind of evil spirit that jumped on a sleeper's chest, so the two words have quite a close conceptual link.