" The Na are a whole different story. The Na have four very slender legs. Sometimes they rear up and walk on just two legs, using the other legs as hands. They have large heads shaped like Andalite heads, only they have just two huge eyes.
Ellimist
--A sentient being with the ability to travel between different dimensions of space and time. Although these peaceful creatures have the ability to foresee and change the future, they are not allowed to interfere directly with the course of history.
Gedd
--A less intelligent quadruped species of the galaxy, the Gedds were the first species to be infested by the Yeerks. Characteristically, Gedds may be seen on infested planets hunched over on all fours, busy at some menial task.
Helmacrons
--the Helmacrons are a race of small creatures bent on taking over the universe. Fortunately, their microscopic size inhibits them from doing so. Nevertheless, they continue to zoom around the galaxy in their toy-sized spaceships, bent on subduing the larger species with their pinprick-sized Dracon beams. Their ships draw power from the blue box, and convert the power into energy that can shrink objects down to their own size.
Hork-Bajir
--Although for all their looks they appear ruthless and deadly, the Hork-Bajir were actually a quite peaceful species before being enslaved by the Yeerks. An average Hork-Bajir is about seven feet tall, dark in skin color, and covered in spikes from head to foot. The Hork-Bajir are the shock troops of the Yeerk Empire.
Kafit Bird
--the Kafit bird, native to the Andalite Homeworld, is usually the first creature a young Andalite aristh morphs. The bird has more wings than would be normal for an Earth bird, making it very easy to spot among, for example, a flock of seagulls.
Leerans
--the Leerans are a froglike species of creatures with the ability to read any creature's mind, while at the same time blocking their own mind from any probing. The Yeerks are especially eager to enslave their entire species, because it would greatly aid in their search for the "Andalite bandits." When a Leeran is infested, the Yeerk slug is inserted in the back end of the end.
Mercora
--Crablike in appearance, the Mercora were dominant during the time of the dinosaurs, and lived underneath a protective bubble in an enormous cavern. This species fought the antlike Nesk for territory, and were the first to cultivate broccoli plants. The Mercora was presumably wiped out during the late Cretaceous period, along with the last of the dinosaurs.
Nesk
--the Nesk were an antlike species in dominance during the late Cretaceous period. Their mortal enemies were the Mercora, who died out around the same time as them. The Nesk were actually a group of sentient ants who were part of a collective consciousness which allowed them to bind together and create larger lifeforms, often fooling their enemies into believing they were bigger then they were.
Skrit Na
--
"The Skrit Na...are actually like two different races. The Skrit look like huge insects, almost as large as an Andalite. They have fourteen legs and six sets of antennae, and aren't really very intelligent. But the Skrit each eventually weave a cocoon and a year later, out of the dead Skrit there pops a Na.
--excerpt from The Andalite Chronicles by K.A. Applegate.
Taxxon
--the Taxxons are a species of centipede-like creatures with many-segmented bodies which they drag along the ground at a rapid rate. Born with an eternally voracious appetite, the Taxxons have been known to eat their own dead. The Taxxons red, teeth-rimmed mouth gives it maximum chewing ability. The Taxxons live in complex catacombs which have been designed for them to slither through. The Taxxons have been infested by the Yeerk Empire.
Veleek
--a "morph hunter." A dust creature from Saturn employed by Visser Three to hunt down the Animorphs by materialising and attacking them whenever anyone of them tries to morph. The Veleek is actually millions of creatures joined together to form a creature of gnashing teeth and slicing blades. The creature feeds on energy and avoids water, which can dissolve it.
Venber
--extinct ice creatures native to the planet Venbea, the Venber were accustomed to living in sub-zero conditions, and would in fact melt if exposed to conditions above freezing. Their characteristic appearance was a silvery body with red and blue streaks across it.
Yeerks
--the Yeerks are a sluglike, parasitic species which originated on an enormous Yeerk pool on their home planet. From there, they infested the Gedds, which are native to their planet. After tricking the Andalite named Seerow into letting them off their planet, they went on to become the Yeerk Empire and are now at work trying to infest the entire galaxy. The Andalites are their mortal enemies. One word sums up a Yeerk's appearance: slug.