Zoids
Zoids make up a large part of Planet Zi's native animal life, and form the basis for most of the mecha critters of the same name. The word Zoid, in fact, is a portmanteu of zoic (that is, having the form of an animal) and android. Due to Zi's high metal content, Zoids are rather metallic themselves, and even wild look much like organic mecha.
Biology
See also: Wild Zoids
Unlike Earth animals, Zoids evolved so that their life force, as it were, was mostly concentrated in cores; the metallic bodies serve more as protective shells and locomotion. Like any other animal, however, they still eat. There are herbivores, which get the required metals and nutrition from plants, and carnivores, which are poor at digesting plants and thus kill and devour other Zoids' cores to get their fill...and, of course, those weird omnivores do both. Dark Zoids are also mentioned as eating an ore called Deochalcum, which served as both power-up and a way to make their underbits glow a fetching shade of neon green, justifying Tomy's use of glow-in-the-dark plastics.
As one might guess from the whole centered-around-core deal, Zoids reproduce asexually: near the end of their life cycle (which can be quite long, depending on the type of Zoid), their core buds off copies of itself. After the parent dies, its body cracks apart, and the cores seek out water to grow metal shells and emerge as proper Zoids. (Note that Zoidians don't do the core deal, and reproduce by shagging.)
History
Zoidians, being Zi's native looks-almost-human humanoids, shared a close bond with Zoids from the start, taming and domesticating them much like humans did livestock and dogs. In the harsh Nature Hates You All environment of early Zi, they were more companions and working animals, but as the planet settled and fighting broke out over territory, Zoids were increasingly modded to be used as living weapons, carrying their buddies into battle and tearing each other apart. Eventually, this would lead to entirely new bodies built up: mecha shells, as it were, to replace the biological ones, with the Zoid's core controlled mechanically. The general idea of a bond between pilot and Zoid wasn't lost, though, and artificial Zoids, as later created by humans, weren't very well-liked.
After the comet-spawned disaster that ended the OJR, many wild Zoids became extinct, and those that remained saw greatly depleted numbers. Zoidians put much of their number under protective management, which both stabilized their population and led to an overall decrease in vitality—some breeds had become rare enough they were revived via cloning. The truly wild Zoids that remained were among the toughest and most successful; they would eventually be used to produce mecha like the Liger Zero, Berserk Fury, Gojulas Giga, and Seismosaurus. These closely-based-on-wild-Zoid designs were extremely responsive, seeming to pick up directly on their pilot's feelings and respond with accordingly better reflexes—another nod to Zoids' animal origins.
OS-enhanced Zoids, an earlier innovation than the Zero family, were more unpredictable about how they called on their animal sides, and ranged from smarter/faster to fiercely independent and bad-tempered. Notable examples would be Gojulas the Ogre, which was nearly impossible to control, but also immune to Dark Spiner's attempts to jam its systems; and the first Death Stinger, which killed large numbers of Zoids on both sides in an attempt to take their cores and make more Death Stingers.
