THE HORK-BAJIR
The Hork-Bajir were a peaceful race despite their fearsome look. This was until the Yeerks enslaved them. Now they are the Yeerks killing machines because although they are naturally peaceful they are able to fight very well as they have a number of blades, are strong and very big. The Hork-Bajir are also, well stupid. They are not very intelligent, they have limited speech (only about 500 words) and cannot write, read or anything like that. Before Andalites arrived at their planet they did not know the concept of fighting, they only learned by copying a fellow Hork-Bajir, Dak Hamee, who was a seer (this happened before the Animorphs were made and Dak Hamee is dead, another seer has come along, Dak Hamee's great granddaughter Toby who was named after Tobias).
A seer is a unique Hork-Bajir. He learns quicker and has intelligence that is almost as good as an Andalite. These type of Hork-Bajir come once in a generation and are usually sent to the Hork-Bajir people for a reason.
The Hork-Bajir were created by a race called the Arn. The Arn arrived on the Hork-Bajir planet and found that the atmosphere was unstable so (the Arn are extremely intelligent in some ways) they created many varieties to stable it. Then they decided they needed someone to care for the trees and they created the Hork-Bajir. The only problem they weren't able to get rid of a few genes and then once in a while a seer would be born that 'would rival the intelligence of an Arn'. The Arn then lived underground and created monsters to keep the Hork-Bajir from 'visiting' them.
After the entire race of Hork-Bajir was enslaved the Yeerks took over the planet but when the Yeerks reached the planet Earth two Hork-Bajir were freed with help from the Ellimist. Tobias found a valley for them to stay in (the Ellimist showed him and the Ellimist kept it hidden) and soon the two began to expand until their was an entire army of freed Hork-Bajir. The valley was were the Animorphs spent their last few months and that was when the Hork-Bajir became involved in the battle.
Below is a picture of two Hork-Bajir
As you can see the Hork-Bajir has blades on their heads, back of legs (near what we call calves) and the back of the arms. All of these blades were once used for stripping bark but know they are used for killing. You can also see they have a long neck with a head with eyes and mouth and whether you'd believe it or not these fearsome looking creatures can smile (although unless you know what they are the smile would probably make you run in the opposite direction to the Hork-Bajir!!! That is of course presuming you haven't already run away!!).