Gobi no irukauma

The Five-Tailed Dolphin Horse (五尾のイルカ馬, Gobi no Irukauma) is a tailed beast sealed within Han from Iwagakure. It’s name is Kokuo. 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-Tails eventually fell into the possession of Iwagakure and was sealed into Han.

The Five-Tails is often portrayed as the fastest of the tailed beasts, and sometimes the most paranoid. It is said to be ’always on the run’.

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