In Egyptian myth, the wife of Ba-nab-djet, Hat-Mehit, was one of this kind of animal, who also appeared on the crown worn by the Hittite goddess Atargatis.  
One of them brought Melicertes's corpse to his uncle Sisyphus.  In Brazilian folklore, the boto is one of these creatures who at night turns into a man to sleep with women, though he never takes off his hat.  In Egyptian myth, the wife of Ba-nab-djet, Hat-Mehit, was one of this kind of animal, who also appeared on the crown worn by the Hittite goddess Atargatis.  A constellation in this form was created in honor of the emissary of Poseidon who convinced Amphitrite to marry him.  More notably, a singer who was travelling from the court of Periander to his home on Lesbos was saved by one of these creatures after his shipmates threw him overboard.  FTP, name this kind of aquatic animal sacred to Apollo, which rescued Arion from a watery grave.
One of them brought Melicertes's corpse to his uncle Sisyphus.  
the dolphin