The origins of the Presa Canario are as old as the legends of the Jardines Hespérides. Many stories have been told about the dogs from the Canary Islands which confuse reality and fiction and experts and historians often contradict each other when talking about the characteristics of the native dog or the role which they played in the Conquest.
The first sign of their existence dates from the 1st century when the philosopher Plinio named the islands after the large dogs which King Mauritania Juba II found on them. From this first mention the conception was born which characterised the former island canines as ingentes magnitudinis. However, in his General History of the Canary Islands, Agustín Millares Torres said more precisely that the first historiographers described these dogs as “wild dogs, which seemed like wolves, but quite small”. Another reference to the size of the dogs can be found in the book History of Our Lady of Candelaria, by Fray Alonso de Espinosa, who spoke of dogs who ate the dead bodies of the Guanches after battles with the Spanish conquerors: “These dogs were terriers or small spaniels which were called cancha, bred by the natives”.
Due to these references, Manuel Curtó García affirms in his dissertation El Perro de Presa Canario: Historia de esta Maravillosa Raza that very little is known about the Canary dogs before the conquest but it is a fact that “their size: small”. Other historians, such as José Juan Jiménez, sustained that the original Canary dogs were confused with a species of large monk seals which populated the coasts of the Archipelago until the 15th century, known as cannis marinus.
Doubts on the physical characteristics of Presa Canario are mainly due to the mixture of races which occured with the arrival of imported dogs from Great Britain and Mainland Spain after the Conquest, and which led to the islands breeding different breeds such as mastiffs, shepherd dogs, Ibizan hounds, pointers, pachons, bulldogs, poodles and sabuesos, among others. For this reason, many experts today still question that the current Presas are descendants of the former dogs. These are questions which will probably never be answered.
Emphasis added by me.
Although the first reference is from the 1st century, so any number of dogs could have been brought to that island before that time, as nick said by boat.
Humans also get to islands by boat. Spiders can fly there and jellyfish die ashore.
Regards,
Daniel