Kokuō

The is a tailed beast sealed within Han from Iwagakure. It has the body of a horse, but the head is based off a dolphin.

Background
According to Madara Uchiha, the Four-Tails came into being near the end of the waning days of the Sage of the Six Paths' life; to prevent it's resurgence, the Sage split up the demon's chakra of the Ten-Tailed Beast into nine seperate, monsterous constructs of living chakra, which would become known as the 'Tailed Beasts'. It has since been captured, extracted, and sealed by Akatsuki.

Trivia

 * According to Kishimoto's words from the second Naruto artbook, he tried to do a mix of a dolphin and a horse.