Quoting the opening caption from Zoids Chaotic Century exactly:
"In the far reaches of the Milky Way, on the Planet Zi, there exist metallic life forms that posses superior fighting skills. These are the Zoids. Warriors by nature, Zoids have been used as the ultimate weapons in an ongoing war between the Imperial forces, and the Republicans of Planet Zi."

Essentially a Zoid is a large, sentient, mechanized being that is extremely loyal to its pilot.

For the full text of all Zoids prologues, see:
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