Legendary Golems is an unofficial term used to refer to the trio of Eronze, Erion and Erace. Other unofficial terms include Titans, Automatons, Eras and Robots.
Each member of the trio is (at least visually) a synthetic, inorganic Pokémon, and is themed after an age of man: Eronze is the Bronze Age, Erion the Industrial Age, and Erace the Space Age. Each member also has a different number of eyes, ranging from 1 to 3, in accordance with the sequence of the ages they are based on, which are located inside an orifice or orifice-like structure on each. Each member of the trio also has similar proportions and body layouts: no distinct head, large upper body and arms, and a small lower body. Lastly, each member of the trio has their elemental affinity shown on or above their backs in their sprites: Eronze has a flame, Erion has clouds of smog, and Erace has electric arcs.
All three golems share the same primary type: Steel. Each of their secondary types is weak to Ground, making the entire trio doubly weak to Ground moves (as Steel is also weak to Ground). This was an intentional design choice, meant to symbolize how humankind is still at the mercy of the Earth, both in terms of the impermanence of our structures and as living things.