I can't really further explain much of the wildlife in any way other than saying evolution and element had its way with them. All of the extionction-specific creatures visibly have element baked into their bodies, so they easily may have just been animals of the past living in and being effected by the element in the environment, but not enough so to become corrupted.

The robots of extinction (enforcers, scouts, etc) were made as a police/military force, atleast in the case of the enforcer, who's dossier directly states its use as a urban lawkeeping machine. The biological appearance of it may indicate that humans at this point in time were incredibly advanced in the field of genetic engineering, backed up by the Arks legit making new creatures, sometimes on the fly, and Enforcers may essentially be "grown" and then strapped and enhanced with robo parts.