Sirens
The tradition of Sirens is related to mermaids, as they are often portrayed as half-woman, half-fish, but sometimes their lower half could that of a bird or an ordinary woman. They are never anything but evil and deadly, and spend their time luring sailors to a watery grave. In Greek mythology, Sirens sing a song which is irresistible to sailors, and causes them to sail towards the sound, only to drown. Naturally, some were curious to find out what this song sounded like despite the dangers. One man who heard the Sirens and lived to tell the tale was Odysseus.
Under the advice of the sorceress, Circe, Odysseus has his sailors plug their ears with beeswax, and ties himself to the mast, instructing his men not to release him no matter how plaintive his requests. He survives the ordeal, as his men were impervious to the song. By the medieval period, literal belief in Sirens was discouraged, and they began to take on an allegorical significance. To the medieval mind, a Siren was intoxicatingly beautiful to the waist, and her song lured sailors and finally sent them to sleep in their boats. The Siren then killed the unfortunate men.
Morally, Sirens as monsters warn of the dangers of succumbing to temptations, and are aligned with the devil because love of worldly pleasures makes the sinner the prey of Satan. Like the mermaid, their presence on maps is a warning of the dangers of the unknown world, but also a warning about the increased temptation of succumbing to pleasure amongst foreign women. After all, non-Christians were seen as essentially evil until relatively recently, and so foreign women of different faiths must therefore be agents of Satan, helping him to seize the soul of the man tempted into committing sin.