The Heralds

The Heralds of Chaos are dark spirits created by Vaatu as vessels of his chaos to spread his corruption. They were also created in order to be able to have enough powers to free him from his prison in the Tree of Time in the Spirit Realm. Each of them has unique, supernatural abilities. Long ago, Avatar Wan sealed each of them within shards of the Chaos Jewel during the Zodiac War. They were eventually lost and forgotten in time as legends and folklore.
In the present day, they are being sought out by The Celestials to unite them to free Vaatu to usher in 10,000 years of chaos once more, more than 400 years before Harmonic Convergence. Avatar Kyoshi is currently looking to possess them and re-seal them within their shards before they can spread corruption and chaos and devour the world with Vaatu.
THE CELESTIALS are known to have four of the shards and Heralds - The Tigershark, Polarbeardog, Dragonlion, and Pumagoat. TEAM AVATAR has four shards and Heralds - The Rabaroo, Elephantrat, Butterflyboar, and Batmonkey. IN THE WILD, four more shards and Heralds are in the wind - The Seahorse, Snakeparrot, Komodorooster, and Eagleox. 

WATER

Itachizame, The Tigershark - A Herald that can inspire rage within people in its vicinity, making people act berserk. It is known to be with the Celestials, who are looking for a new host to possess it with. 
Nanook, The Polarbear Dog - A Herald that can empower and enhance those around it with its howl. It is currently known to be with the Celestials, and the current host is Niya.
Kuda Laut, the Seahorse - A Herald who can manipulate the weather. The Fifth Nation has this shard hidden within their vault on a secret island.

EARTH

Bilis, the Rabaroo - A Herald who can make vegetation sprout anywhere. Its former host was Don Malakas, but has been re-sealed in its shard. Bilis wanted full autonomy and wants to rule the world with the Repolyo Crime Family. The shard is in the possession of Kyoshi.
Okizumi, the Elephantrat - A Herald who can travel through space with the use of portals. Its previous host was Master Jianyu, but the shard has been re-sealed by Kyoshi and is in her possession. Okizumi wants full autonomy but is a coward as it is the weakest of all Heralds..
Ahaloro, the Snakeparrot - A Herald whose venom can spread an infectious disease or heal any ailment. Its last known location was somewhere in Ba Seng Se.

FIRE

Mang Kong Sang, the Dragonlion - A Herald whose fire can destroy anything and wipe it out of existence, banishing them to the spirit realm. It is currently known to be with the Celestials.
Rinca, the Komodo Rooster - A Herald who can copy the powers of any of the Heralds with just one touch. It is thought to be in the possession of the Sun Warriors, in the Fire Nation.
Bakree, the Pumagoat - A Herald who can give the beholder supernatural strength and invulnerability. It is currently known to be with the Celestials with Qi'rah as its host. 

AIR

Amihan, the EagleOx - A Herald who can travel through time. It is currently dormant, thought to be hidden in one of the Air Temples.
Liang Zhu, the Butterflyboar - A Herald who can negate the effects caused by the powers of other Heralds. Its presence also negates its influence of chaos. Liang Zhu wanted to create a perfect, peaceful society through the Air Nomads by any means. Rishi was its previous host.
Pikling, the Batmonkey - A Herald who can control those around it with telepathy and becomes stronger as deaths happen around it. Meenu/The Ember Island Murderer was its previous host but it has been re-sealed and taken by Kyoshi. It just wants to murder people, that's all.

Summary:
With the Celestials: 4 (Tigershark, Polarbear Dog, Dragonlion, Pumagoat)
With Kyoshi: 4 (Rabaroo, Elephantrat, Butterflyboard, Batmonkey)
Unknown: 4 (Seahorse, Snakeparrot, Komodo Rooster, EagleOx)