Dolphinmer have the upper body of a human and the lower body of a doplhin. They are frightinging agile in the water, and exceedingly playfull. They tend to be on the smaller side of most merfolk, though streamlined, and pretty much perpetually youthful in appearance. gonna be in book 3 check out this link to learn all kinda things about them to help you understand the book This link!
These are orcans! Orcans, as stated, have the upper torso of a human and the lower body (tail) of an Orca. They are typically large, often of a size with Tritons and about as strong. They`re gonna be in book 3 to! So learn more here
Learn about Octomers here
Learn about tritons here