E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
So called from monteros dEspinoza (mountaineers), who once formed the interior guard of the palace of the Spanish king. The way they came to be appointed is thus accounted for:Sanchica, wife of Don Sancho Garcia, Count of Castile, entered into a plot to poison her husband, but one of the mountaineers of Espinoza revealed the plot and saved the counts life. Ever after the sovereigns of Castile recruited their body-guards from men of this estate.