The Hork-Bajir are usually about seven feet tall. They are a dark shade of green and have three-toed theropod feet. Their necks are long and they have a mouth that resembles a falcon's beak with teeth. Their legs are long and their knees bend backwards. They have blades at the wrist, arm, knees, and forehead; plus spikes on the tail. Marco describes them as "walking salad shooters" when they are first seen in the first book because of their spikes. Their horn-like blades vary by gender, males having three and females having two, although artwork in the series does not convey this. The Hork-Bajir were taken over completely and utterly by the parasitic Yeerks. Despite their fearsome looks, they are peaceful herbivores that utilize their blades to harvest tree bark. Different blades have specific jobs for cutting bark. Elbow blades for a straight cut, wrist blades for taking strips of bark off, and knee blades for cutting bark close to the ground.
Unlike the traditional idea of aliens, they are not super-advanced; in fact, the average Hork-Bajir intelligence is said to around the level of a four-year-old human, except for seers (see below), though they are still considered sentient. Even with Yeerks under their control Hork-Bajir seem to have mental limits; Hork-Bajir-Controllers often speak in a mixture of human and alien languages, which is explained as being because the Hork-Bajir brain has limited linguistic abilities (their own language is believed to be very limited). They possess no technology of their own. Hork-Bajir do, however, have incredible endurance and healing ability. One, named Jara Hamee, once sliced his very head open, exposing his brain, to demonstrate the absence of a Yeerk in his head. It immediately scarred over afterwards.
The Hork-Bajir lifespan is unknown, but is apparently shorter than that of humans or Andalites. It also seems that they grow to adulthood very quickly, as Toby Hamee was born and grew to near-maturity in no more than a few years; The Hork-Bajir Chronicles also mentions that Hork-Bajir choose mates sooner than Andalites do.
In combat, an Andalite tail is faster than a Hork-Bajir blade, although Hork-Bajir warriors still pose a great threat to their Andalite counterparts and they are the main shock troops of the Yeerk Empire.
Hork-Bajir did not in fact evolve naturally: they were genetically engineered by the original dominant race on their planet called the Arn. After an asteroid struck the planet several millennia ago, most of the planet was left uninhabitable apart from deep valleys that were ripped open by the impact. The Arn, who were adept at genetic engineering, created enormous trees with which the atmosphere could be maintained in the valleys. Not willing to become a race of tree-herders, they created the Hork-Bajir as being naturally suited to tree-herding. So that the Hork-Bajir remained unaware of their existence, the Arn also created a layer of blue gas halfway down the valleys: the Hork-Bajir lived above, the Arn lived below, and several types of genetically engineered monsters lived within the blue mist to scare away any Hork-Bajir.
When the Yeerk forces discovered the Hork-Bajir homeworld, they saw not a race of tree-herders but shock troops. They began to kidnap and take the Hork-Bajir as hosts, at first covertly but later openly. For seven months a guerilla resistance was headed by the Hork-Bajir Seer Dak Hamee and the Andalite Aldrea, until the Andalite forces arrived. But even then the reinforcements were small and ineffective. The Andalites and Hork-Bajir were losing the war fast. Eventually the Andalites---lead by Alloran---used a quantum virus to kill the majority of the Hork-Bajir population in an attempt to prevent the Yeerks from taking more as hosts---an act considered to be a terrible crime, but which did manage to limit the Yeerks' power. Consequently the Hork-Bajir now exist in limited numbers.
The Hork-Bajir were totally enslaved for several generations, until the Ellimist managed to free two on Earth, a married couple named Jara Hamee and Ket Halpak. Tobias and the other Animorphs managed to hide the two in a valley, which, due to the Ellimist's powers, was very hard to find. These two soon had a child named Toby (see "Seer" below), and began to liberate other Hork-Bajir from Yeerk facilities. This became the Free Hork-Bajir colony, a resistance movement against the Yeerks. Later, the last living Arn came to Earth and took DNA samples of the colony, and returned to the Hork-Bajir homeworld, using cloning techniques to create a second free Hork-Bajir group there.
When the Yeerks were defeated and driven from Earth, the Hork-Bajir were given Yellowstone Park as a new place to live, and Toby Hamee became a non-voting member in the U.S. Senate. It is unknown what became of former Hork-Bajir-Controllers or the Hork-Bajir resistance on their home world.
On their home world, the Hork-Bajir lived in tribes which coexisted peacefully with each other. The Hork-Bajir have families consisting of a pair of life-long mates and children, though the tribal structure seems to make the family structure fairly loose.
Hork-Bajir naming seem to be similar to those used by humans in the Western hemisphere, a given name followed by a family name. Female Hork-Bajir apparently keep their own family names after choosing a mate, though children seem to inherit their father's name (for example, Jara Hamee and Ket Halpak's daughter is Toby Hamee). The Hork-Bajir have their own language, though according to Ax, it is extremely limited, and the Hork-Bajir brain is said to have limited linguistic abilities; as a result, they do not speak English well, and Hork-Bajir-Controllers are known to speak in a mixture of English and alien languages, including Galard.