Metagross did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Metagross, known in Japan as メタグロス (Metagross), is a special Steel/Psychic-typePokémon. Having evolved from Metang at Level 45, Metagross is the final form of Beldum. It is considered a pseudo-legendary Pokémon, despite not looking the slightest of draconic, which most pseudo-legendary Pokémon have in common with each other. Metagross is considered as the signature Pokémon of Steven Stone, champion of Hoenn. Metagross is a genderless Pokémon.
Metagross can Mega Evolve into Mega Metagross when it holds a Metagrossite.
Metagross is a pale turquoise, metal Pokémon that consists of a disc-like body and four enormous legs. Its body doubles as its head, and its overall construction makes it resemble some kind of arachnid. The upper part of its body consists of two black holes with red eyes inside, and a silver cross. The position of the cross makes it resemble a mustache and a monobrow at the same time. The upper part is also where the joints to its legs are located, which make use of a ball-and-socket kind of joint. The lower part of its body is basically a mandible, and is thus where its mouth is located. Two spikes are also located here. The legs resemble the main body of a Beldum. The upper part of the legs are thin with a blunt protrusion, and the lower part is thick, which ends with three claws. These legs can be tucked in so Metagross can float in the air.
Metagross appears to be a fusion of two Metang, and since Metang is a fusion of two Beldum, this gives Metagross a total amount of four brains, joined together by a complex neural network. Because of this, Metagross is able to easily solve difficult calculations faster than any supercomputer could. It uses calculations to analyze its opponent. When Metagross is hunting, it pins its prey right under its body and eats them alive with its mouth.
When it Mega Evolves, its old legs are found on the back, on both on the upper and lower body and tucked in, making room for four new arms with larger claws useful for attacking. The cross on its body becomes larger and gold-colored. The spikes on its lower body become two triangles each, and are placed between the upper and lower body. Its mandible gets two holes, and there's a humongous spike placed on its chin. All of its stats gain a boost when Mega Evolved, and Mega Metagross is more balanced than ever.