The Zoidians (sometimes called Ancient Zoidians) were the original inhabitants of Planet Zi.
In appearance, Zoidians look identical to normal humans, but have particularly brightly-colored eyes and hair. In the Battle Story, some Zoidians have metallic parts over their bodies, while in the anime, some have been known to possess unique abilities similar to psychic powers. Zoidians live much longer life spans than regular humans.
Zoidians originally appeared in the Battle Story text as native inhabitants of Zi. They fought using primitive Wild Zoids and formed clans, including the progenitors of the Helic Republic and Guylos Empire. The arrival of humans on Zi caused a massive arms race on both sides, with human weapons and technology greatly advancing the wartime capacity of Zoidians.
Many Zoidians were wiped out during the catastrophe that decimated Plant Zi.
The Zoids: Chaotic Century/Guardian Force anime (and manga) are roughly set after the catastrophe seen in Battle Story. But despite having common background and characters, they story of all three diverge significantly as each progress.
Zoidians were always accompanied by an organoid, and the two went everywhere together. Their technology was far beyond that found on modern Planet Zi, among their various achievements the Zoidians possessed skyscrapers, universal sources of energy, and advanced Zoid engineering skills.
It is practically impossible to describe their way of life, because of the lack of knowledge upon the current residents on Zi, and the fact that the Zoidians that do appear in the series either do not remember the past, or never talk about it.
In Chaotic Century/Guardian Force, it appears that Zoidians have senses that are substantially more acute than a human's. Such instances are mostly seen through Fiona, who is capable of hearing and smelling water from significant distances that escape human detection. As well she has notably enhanced vision, as she was able to rapidly spot out and calculate the number of threads on a loom from where Van's sister made a mistake, which were in the count of thousands in just the span of mere seconds.
There is a striking similarity between the ancient Zoidians, and the ancient race in Zoids: Genesis. Both races had similar technological prowess, and both were left practically extinct due to some disaster. However, the ancient race in Genesis is not called Zoidians, and are not referenced as being natural inhabitants of Planet Zi.