Cosmog resembles a floating cloud colored in dark blue and purple. Two light-blue cloud-like formations stem upward from Cosmog's body and seem to be similar to a pair of arms. It has a gold crescent-shape around its body and its face is black with blue cheeks.
As seen in Lillie's Cosmog Nebby, it appears to be a curious and playful Pokemon. It also is overly trusting and naive, carelessly approaching even people who may not have the best intentions.
In the anime, Nebby behaves like a baby spending most of it's time crying and sleeping. But it's also curious and mischievous when using it's teleportation powers.
Under stress and pain, Cosmog can create Ultra Wormholes in an attempt to escape its tormentor(s).
Cosmog evolves into Cosmoem, which evolves into a different form depending on the version of the game. If it evolves in Pokémon Sun, it will become Solgaleo; whereas if it evolves in Pokémon Moon, it will become Lunala.
Cosmog is the first evolutionary form of Solgaleo or Lunala. Cosmog can evolve into Cosmoem at level 43 which evolves into Solgaleo (Sun) or Lunala (Moon) at level 53.
A Cosmog named Nebby features prominently in the games' main plot, under the ownership of Lillie. A running gag in the story is Nebby frequently escaping from Lillie's handbag and getting itself into trouble, hence spawning the Internet meme "Get in the Bag, Nebby". Another Cosmog nicknamed Nebby will appear in the anime in SM044, and it is probable that Cosmog will stay with Ash instead of Lillie.