What kind of animal is dolphin?

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Suborder: Odontoceti
Family: Delphinidae

Dolphins, along with whales and porpoises, are thought to be descendants of terrestrial mammals, most likely of the Artiodactyl order. Dolphins entered the water roughly fifty million years ago. Dolphins have a streamlined fusiform body, adapted for fast swimming. The basic coloration patterns are shades of gray with a light underside and a distinct dark cape on the back. It is often combined with lines and patches of different hue and contrast.
Modern dolphin skeletons have two small, rod-shaped pelvic bones thought to be vestigial hind legs. In October of 2006, an unusual Bottlenose Dolphin was captured in Japan that had small fins on either side of the genital slit, which scientists believe to be a more pronounced development of these vestigal hind legs.
The head contains the melon, a round organ used for echolocation. In many species, the jaws are elongated, forming a distinct beak; for some species like the Bottlenose, there is a curved mouth that looks like a fixed smile. Teeth can be very numerous (up to two hundred and fifty) in several species.
The dolphin brain is large and has a highly structured cortex, which often is referred to in discussions about their advanced intelligence. A recent theory however disputes the existence of a neural basis for dolphin intellect, suggesting instead that the large brain is only an adaptation to living in cold water. This theory has not found widespread acceptance.
More informations:
http://en.wikipedia.org/wiki/Dolphin