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. It was later resealed into a resurrected Han during the Fourth Shinobi World War.

Background
According to Tobi, the Five-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his ability 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 with a dolphin's head. It has two pointed long horns and two short horns. The ends of its horns, hooves, and tails are light brown, with some of the same-coloured spots before the brown areas of its horns and hooves. It also has red markings under its dark blue-green eyes, similar to Kimimaro's.

Abilities
Not much is known about the Five-Tails, but since it is a tailed beast, it has a great amount of chakra in comparison to most shinobi and is able to use the Tailed Beast Ball. The Five-Tails can also use its horns in combat, showing enough ramming-force to even injure the Eight-Tails.

Fourth Shinobi World War Arc
After Han's full transformation into the Five-Tails, he rammed into the Eight-Tails and pushed it a considerable ways back.

Trivia

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