“Un nuevo sustantivo latino en Isidoro de Sevilla (Orig. XII,1,55), mauro, mauronis, ‘caballo moro’ étimo del español medieval morón”, La Filología Latina: mil años más. Ed. P.P. Conde Parrado / I. Velázquez (Burgos: Instituto Castellano y Leonés de la Lengua, / SELat 2009), 1, 165-183

The reading mauro  in that text of of Isidore's Etymologies is not the dative of the adjective maurus, but the nominative of the third declension of a noun referring to the Moor or Berber horse first, and later perhaps also to horses of another geographical origin with similar morphological characteristics. This term mauro may be the origin of the medieval Castilian word morón referred to a type of horse.