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"The Arn who survived needed trees to provide oxygen. Lots of exceedingly large trees. They didn't feel like taking care of the trees themselves. Solution? They used genetic engineering to design creatures of low intelligence who ate tree bark: the Hork-Bajir. An elegantly simple solution for the Arn who were masters of genetic manipulation."

The Arn were the original inhabitants of the Sector 5, RG-21578-4. Masters of biological and genetic engineering, the Arn created and planted the trees on the planet, and created the Hork-Bajir to tend to the trees as well as monsters to keep the Hork-Bajir unaware of their existence.


"The Arn stood in the center. The first thing I noticed about him was his eyes. They glittered like diamonds lit from within. Their intensity dazzled me. I blinked a few times, and began to take in more details of the Arn's appearance. He had four legs, two elongated arms, and a pair of short wings. He was about half as tall as Ax and his skin was a vibrant emerald-green."

The Arn were an avian race that originally inhabitants of the Hork-Bajir homeworld. They have four legs, short wings, and two elongated arms ending in claws. They are covered in brightly colored feathers vary wildly in shades. Their faces are dominated by glittering eyes and small, red mouths.[1][2]


"We returned to our home world to find everyone dead. Our entire species. The air was unbreathable, except in the valleys. But even there, the balance was precarious. A hair too much carbon dioxide, a shade too little nitrogen, and even the impact valleys would die. So we went to work to understand this new environment. We needed a mechanism for controlling the atmosphere."
"The trees."
"Yes, of course. The trees. Different species, each subtly different in its use of carbon dioxide and its production of oxygen. The perfect balance, the perfect mix, that's what we needed. But they would require constant care. And we were not willing to become a race of tree-herders."
Quatzhinnikon and Aldrea-Iskillion-Falan[src]

The Arn were masters of biological innovation, so much so that they had little ability or interest in traveling past their own moons.[1]

After an asteroid collided with their world and destroyed much of the life of their planet, the Arn created an entirely new ecosystem to keep the planet alive. The Arn then decided to shield themselves from any more such catastrophes by moving their settlements into the deep chasms caused by the disaster.[1]

They created the Hork-Bajir in order to care for and support the vast jungles of high-rise trees. The Arn also created various creatures to ensure no intrepid Hork-Bajir entered the Arn settlements deep in the planet's chasms, which was known to the Hork-Bajir as Father Deep. The Arn controlled these creatures with chips implanted in their heads, which allowed them to give the creatures simple commands.[1][2]

Andalite-Yeerk War

Assisting the Hork-Bajir Resistance

"It was a sad, sick collection. In a better world, a world of peace and justice, someone would have punished the Arn for what they had done. Twisting life to make monsters is an evil thing to do. But their evil served our purpose. We had an army."

To be added

Arn Slavery

"We hid among the Arn, for the most part. The Arn didn't like it, but they were helpless. Of course, the secret of the Arn was now well-known to the Yeerks. As Quatzhinnikon had predicted, the Yeerks discovered they could not successfully infest the Arn. So the Yeerks used the Arn in other valleys as slave labor to mine their raw materials and to build Yeerk ships. When an Arn was injured or worn out, the Yeerks used them for target practice. But the Arn in my valley were untouched."
Dak Hamee[src]

When the Yeerks invaded the Arn/Hork-Bajir homeworld, the Arn denied the Yeerks any possibility of being made into Controllers, or any Yeerk acquisition of the Arn's advanced technology. They achieved this by altering their body by making an already weak blood vein even weaker, so that when a Yeerk enters the brain, the vein will pop, thus killing the would-be host and Controller. Upon realizing this, the Yeerks used the Arn for slave-labor and target practice and eventually exterminated them through mass murder, exhaustion, and starvation.[1]

Soliciting the Animorphs

"Alien come valley."
"The Yeerks? They found you?! Did they attack you? What's the situation?"
"Not Yeerks. Arn. From the old world. Arn... make... Hork-Bajir."
Jara Hamee and Cassie[src]

Quafijinivon, one of the few surviving Arn, traveled to Earth on a stolen Yeerk light freight ship and studied and absorbed every human language. He found the Hork-Bajir valley and the Animorphs. There, Quafijinivon used a technique known as ixcila, whereby a dead individual's 'soul' or 'essence' would find a host and become alive again. In this case, that person was the Hork-Bajir nothlit Aldrea, Seerow's daughter. Quafijinivon solicited the Hork-Bajir and the Animorphs' help, and they traveled to the Hork-Bajir world to help the dying Arn create Hork-Bajir in his lab.[2]

Cassie had to share her mind with Aldrea's memory so they could find a stash of weapons that Aldrea had hid on the Hork-Bajir homeworld. This also allowed them to read each other's thoughts and memories.[2]

Known Arn