The Iberian zebro: what kind of a beast was it?

Seven centuries before the discovery of the African zebras by the Europeans, the names zebro and zebra were given to an enigmatic equid widely reported in the Iberian Peninsula during the Middle Ages. Roughly 150 toponyms deriving from the words zebro/a have been recorded in Iberia starting from the IXth century A.D. together with 65 Portuguese Forais and 16 Spanish Fueros (i.e. town laws) referred to these animals as onagri or zebros – Latin and Romance etimology respectively – during the XIIth and XIIIth centuries. However, the precise biological nature of the animal remains unclear to this day. Four hypotheses have been put forward to explain the taxonomic status of the zebro: 1) Equus hydruntinus, an extinct onager; 2) the antecessor of the Sorraia horse breed; 3) a modern onager species introduced by the Muslims; and 4) a feral domestic equid, either an ass or a horse. This paper addresses the issue by reviewing the historical and nomenclatural data on the subject, while briefly covering the genetic and archaeozoological evidence that may eventually shed light on the issue.