GIants were well known for eating children long ago. Coyote encountered one of these said giants one day and decided to teach him a lesson. He got the giant to build him a sweat-lodge and said it would make him as quick and agile as Coyote himself.
A Siren isn't really a sea monster but sailors have told tales of these beautiful women that sing to you and seduce you into their trap of bloodlust. They are said to eat the sailors alive or drowned them deep in the ocean.