What do marshals and mares have in common?
Both of these words derive from a Proto-Germanic word for "horse".
The distant ancestor of 'marshal' was *marhskalk, which meant "horse attendant", so 'marshal' has made quite a career!
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The word 'nightmare' is unrelated to 'mare' (horse). This '-mare' comes from Proto-Germanic *marōn (evil spirit).
However, coincidentally, the Late Middle Dutch cognate 'nachtmare' (nightmare) was folk-etymologically conflated with 'merrie' (mare ) and became 'nachtmerrie', the current Dutch word for 'nightmare'.
As the stand-alone word 'mare' meaning 'evil spirit' had become obsolete, the part '-mare' didn't make sense anymore and was supplanted by a more transparent word: 'merrie'.
@yvanspijk mare is in het Nederlands toch nog steeds 'slecht nieuws'
@BasieP Dat is weer een ander 'mare'. Dat komt van *mērijan (verhaal), waar via-via ook 'vermaard' van afstamt.
@yvanspijk @BasieP And that's also the source of Mär- in Ger. Märchen (a little Märe 'story, account'). The related OHG adjective māri also meant 'famous'.