Many people have commented on the mouth of the Spanish Sulphur breed and have often incorrectly referred them to be "parrot mouthed" due to the fact that the upper lip slightly overhangs the lower. In reading Juan Llamas' book titled This is the Spanish Horse we find this description fitting of a true Spanish horse. He quotes Joseph de Arcos who published a book titled Comentarios a la Real Ordenanza de Cavalleria del Reyno in 1757 defining the mouth of the Spanish horse as "large [in length] and proportionally long; fine lips, the upper one overhanging the lower".
So, we find that even centuries ago, this ideal mouth that is now relatively unknown in the American Iberian horse enthusiast population is actually the ideal Iberian mouth that has always been present in the old Iberian Spanish Sulphur population. We also come to find that the "Spanish" mouth preferred by these people that are unfamiliar with the true Iberian mouth often prefer a mouth that is referred by the Spaniards as a "calf's nose" that is often present on horses that have draught breeding or on draught breeds as pointed out by Juan Llamas who is a well respected expert on the Iberian horse.