Kokuō

The is a tailed beast sealed within Han from Iwagakure. It has been captured, extracted, and sealed by Akatsuki. In the anime, the Five-Tails is indicated to have been one of the two tailed beasts that Akatsuki captured before the start of Part II.

Background
According to Madara Uchiha, the Five-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tailed Beast's resurgence, the Sage split up its chakra and used Izanagi to create 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.

Appearance
The Five-Tails primarily resembles a white horse, but its head is based off of a dolphin with two long and two short horns. It also has red markings under its eyes similar to Kimimaro.

Trivia

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