Seikishin - also known as Sacred Mechalords in the Funimation translation - are ancient and powerful humanoid constructs that were built by an ancient race called the Senshi.
Seikishin were originally created for experimental purposes, and to show off technological ability. In time, they became used in sports and other forms of competition, including combat.
Gaia, the only surviving Seikishin, was unearthed intact (although immobile) by the Shitoreiyu Empire, and was mainly used as a decoration before the arrival of Kenshi triggered a response from it. It was revealed that the Senshi civilization was completely destroyed by Gaia.