cool find.
Perhaps sailors saw strange animals and such on the islands and thought that they were demons. Islands have much stranger and wilder animals than the
mainland sometimes, such as the Galapagos Islands.
Great white sharks tend to stay near islands too, because of easy prey like seals. When they attack seals, they breach. Breaching is when they thrust
their bodies upwards and completely leave the water for a second:
Plus, sailors and travelers back then were known to exagerate what they saw, so when a map maker heard of these horrible tales he could only find one
word to fit these descriptions:
demon