Zoanoids were human beings that had been optimized for battle so that they could participate in the Creators' war against an unknown foe. Following the failed experiment of fusing a Guyver unit to a human, the Ouranus abandoned Earth and all their creations. Over the millenia, Zoanoids intermingled with normal humans their bloodlines were diluted. Only a few rare Zoanoid subjects would surface over the centuries, becoming the basis of various legends and stories of monsters such as the Yeti, Werewolves and others.
At least 400 years prior to the modern era depicted in the Guyver series, the Zoalord known as Archanfel (one of the original creations of the Ouranus) founded the secret society of Cronos and began inducting human beings into his organisation by turning them into Zoanoids. Between the 1950's and 1960's, Cronos discovered an alien relic in Japan: a lost Creator spacecraft containing all manner of genetic engineering technology. They decided it was in mankind's best interest to continue the Creators' work and began working towards dominating the world through the zoaforming of Earth's population.
Later in the series, despite the efforts of the Guyvers and their allies, Cronos conquers the world and establish a new global government. They encourage the public to apply for Zoanoid optimization, since Zoanoids possess superior physical prowess and are immune to disease. While they do not force zoaforming upon the populace, they also withold the fact that Zoanoids are genetically programmed to obey the psychic commands of the Zoalords, Cronos's leaders.
The enhanced-muscle class Zoanoids are the most common zoaforms and are typically used as Cronos's ground troops. Their muscle power far exceeds that of normal humans and conventional weapons are useless against them.
Bio-blaster type Zoanoids are physically weaker than the power types, but are equipped with the ability to fire lasers that are biologically generated within their bodies. Only a handful of bio-blaster types exist.
Zoanoids adapted for underwater combat.
Weaker Zoanoids that specialize in reconnaissance and observation.
Zoanoids optimized for flight and aerial combat.
"Guyver-Killers" refers to the Enzyme series, a breed of Zoanoids designed for the sole purpose of destroying the Guyver. Originally developed by Cronos Japan, the Enzymes' blood carries a corrosive toxin that will swiftly dissolve bio-boosted armour.
Lost Numbers are experimental Zoanoid models created by Cronos's co-founder, Dr. Hamilcar Barcas. They are considered defective prototypes and were over-optimized to give them more powers than most Zoanoids are able to possess. As failed test subjects, their designs cannot be duplicated.
Hyper-Zoanoids are advanced zoaforms created by Cronos rather than the Uranus. They are a recent phenomenon, having only been introduced following the activation of the Guyver units (though the planning stage for Hyper-Zoanoids was likely at fruition before the first unit was activated). These advanced Zoanoids are all one-of-a-kind specimens, optimized with great care and close observation. They tend to function as generals or intermediaries between the Zoalords and standard Zoanoids and the most advanced Hyper-Zoanoids are even capable of confronting the Guyver on more or less equal terms.
While each Zoanoid model has its own abilities and statistics, there are some common traits shared among all of them. First and foremost is their total obedience to the Zoalords; all Zoanoids produced by Cronos are subject to the Zoalords' telepathic commands and are unable to disobey. Secondly, when Zoanoids die, their bodies immediately disintegrate, leaving no evidence that they ever existed. Before Cronos rose to power over the whole world, Zoanoid disintegration was essential in order to prevent the public becoming aware of them and to avoid complications with the authorities.