Organoids are legendary, wild, living zoids. They're much smaller than combat zoids, about the height of a typical man. Unlike zoids, organoids need no pilot and are more like a pet or friend rather than a vehicle or comabat unit.
Organoids have a very special ability that can be essential in determining the outcome of a battle. They merge with the zoid core to increase the combat ability of a zoid and give it an extra boost of power. However, if a zoid core is destroyed while an organoid is merged with it, there is little chance that the organoid will survive. Organoids can also become gravely injured even if the zoid core remains intact. Harsh combat can become too much of a strain on an organoid and they can be thrown out of the zoid and possibly destroyed. It is possible to revive a fallen organoid with a piece of Zoid Magnatite. If the bright blue rock is placed in an organoids mouth quickly enough, the organoid can completely recover.
In rare cases, an organoid has been know to be able to repair a damaged zoid and bring the zoid back to life. Another rare event is the evolution of zoids. An organoid can wrap itself around a zoid core and en-shield the entire zoid in a cocoon like manner to completely evolve the zoid into a new, more powerful zoid.
In Zoids: Chaotic Century there are four organoids. Zeke, Shadow, Specular, and Ambient. I'll get their images up as soon as I can, but for now enjoy these running gifs of them.
Organoids
Zeke
Master: Van
Fuses with: Shield Liger/Blade Liger
Zeke is very friendly. He resurected the dead Shield Liger that he and Van found abandoned in the desert. Zeke is Van's best friend, and always does what he can to protect Van and rescue him from sticky situations. Zeke has the special ability to evolve zoids into a more powerful form.
Shadow isn't a very nice organoid at all. He shares the deep hatred for Van as Raven has. Shadow is the only organoid with actual wings. All other organoids simply have thrusters on their backs that allow them to sort of fly. Like Zeke, Shadow has the special ability to evolve zoids.
Specular is a very interesting organoid. She can't make zoids evolve, but she has an ability that no other has. She can transmit sound waves that make people see hallucinations. She does a very good job of screwing with people's heads.
Ambient
Master: Hillz
Fuses with: Death Stinger
Ambient seems to be a typical organoid with no real special abilities.