Tapu Koko 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):
Tapu Koko, known in Japanese as カプ・コケコ (Kapu Kokeko), is a possible Legendary Pokémon of the Electric/Fairy-type. It is one of four Alolan guardian deities, protecting Melemele Island.
Tapu Koko is a small, soot-black and orange Pokémon with a slight humanoid shape, having a wide black torso with white markings, yellow shoulder rings, and an orange lower half with four large protrusions. Its black head is dumpling-shaped, with four more white stripes (two of which appear to be eyebrows), and red and blue eyes with black irises and sclera, and yellow side rimming. It also sports a very large, orange comb that runs from the backside of its neck to its chin, giving Tapu Koko the appearance of a rooster, complete with beak-like plumes. The top of its comb is also slightly colored yellow.
Tapu Koko also has two arms with pincer-like hands, each marked with a white and orange triangle. Attached on these hands are two large wooden shells, both shaped like two halves of an egg. These shells are bright yellow and rimmed with black. On each half are three triangles and two rhombus-shaped eyes painted, and each half is also adorned with two wing-like attachments on top. If Tapu Koko conceals itself inside the two shells, this wooden structure will resemble an egg-shaped chicken, with Tapu Koko's hands as a black beak, its orange bottom as tail plumage, and its orange comb sticking out.
Despite it being a guardian, it is quite a fickle Pokémon and doesn't always respond to any call for help. However, Tapu Koko is a curious Pokémon that, when interested in a person or Pokémon, may come to play or battle after all. Tapu Koko encloses itself in its shells in order to store electricity, but also masks itself like that during a special event.