Does it need to be a dolphin? 

Let's say: Goblin shark here is a photo  http://www.sharksider.com/images/goblin-shark-1.jpg

>Goblin Sharks are one of the scariest looking sharks in the ocean. They have powerful retractable jaws, lined with sharp teeth, that jut out and quickly snag their prey. However, not only are they not a particularly aggressive shark, they hunt in very deep ocean waters at a minimum of 330 ft (100 m) so very few humans ever encounter them in the wild. To date, there are no documented cases of a Goblin Shark ever attacking a human.