Kokuō

The is a tailed beast sealed within Han from Iwagakure. It has the body of a horse, but the head is based off of a dolphin. It has since been captured, extracted, and sealed by Akatsuki.

Background
According to Madara Uchiha, the Five-Tails came into being in the waning days of the Sage of the Six Paths' life; to prevent its resurgence, the Sage split up the chakra of the Ten-Tailed Beast into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Five-Tailed Dolphin Horse eventually fell into the possession of Iwagakure and was sealed into Han, but was captured and extracted 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.