The no-mans-land (up to 100km across) between Christian Spain and Moorish-occupied Spain was too insecure for arable farming and was only exploited by shepherds. When the land was reconquered by the Spanish, farmers began to settle and disputes with pastoralists were common. The Mesta can be regarded as the first, and most powerful, agricultural union in mediaeval Europe.
The kings of Castile conceded many privileges to the Mesta. Even today, herds of sheep may be transported by rail, but the cañadas (traditional rights-of-ways for sheep) are legally protected "forever" from occupation and barring.
Some Madrid streets are still part of the cañada system, and there are groups that organize sheep transportation across the modern city as a reminder of old farming.
The word mustang comes from mesteño or mestengo ("a mesta [i.e. ownerless] beast").
Mesta is also a medieval village in Chios, a Greek island in the northern Aegean.