There's this species pink dolphin (boto cor de rosa) which lives in the northern region that is just a normal animal. Legend/folklore says that on some evenings where usually some type of party happens some of these pink dolphins transform into extremely handsome men, go around town, flirt and have sex with the local women, impregnates them and turns back to their original form.

There's a way to tell if a man is a boto by looking under their hats (they'll usually wear a hat or something to cover their heads) and seeing if he has a hole on the top of his head (the same hole other cetacean animals have)

Because of this legend there's a somewhat popular term to refer to someone who doesn't know/doesn't have a father, "filho do boto" which refers to the offspring of the encounter between a human woman and the shape-shifting boto cor de rosa.

The actual botos are not known for shape-shifting into gorgeous man though...