It is not known to evolve into or from any other Pokémon.
It is one of the Ultra Beasts and is known by code name UB-01 Symbiont (Japanese: ＵＢ０１ Ｐａｒａｓｉｔｅ).
Nihilego is an Ultra Beast which resembles a jellyfish. It is mostly white and has tentacles shaped like the hair, dress, and legs of a girl. The top of its head is a bell that resembles a sun hat. The bell is transparent save for six pointed stars around the crown, and the brim has a blue edge. The ends of the "hair" fly out, resembling arms, and the outer tentacles of the "dress" have two bubbles inside.
Nihilego is a parasitic organism, capable of injecting a neurotoxin that subtly alters the thoughts and actions of any human or Pokémon that becomes its host. This neurotoxin stimulates intense feelings of excitement while causing a loss of inhibitions, which acts to forcibly bring out any latent potential within the host while wearing away at their minds.
Nihilego's exact levels of intelligence are unknown, but according to the Aether Foundation's research it seems to act mainly on a self-preservation instinct. It has been observed behaving like a young girl, though the exact reasons for why it does so are unknown.
As shown with Lusamine, Nihilego is capable of physically merging with a host. In this "merged" state, the bell loses its stars, developing several iridescent spots across its surface; the host's body is visible within the bell, with their legs dangling beneath it. Its eight tentacles become long and flattened in appearance, and what appear to be eyeballs with colored diamond-shaped "pupils" cover their surface, along with smaller multi-colored dots. Four of the tentacles are connected to the "hat" via frilly white structures; these tentacles have four spines protruding from them that can be used similarly to hands. The remaining four tentacles each terminate in a single smaller spine. While this merger can be disrupted, the lingering toxins and the shock of the separation are likely to leave the host in critical condition, both physically and psychologically.
This Pokémon was unavailable prior to Generation VII.
Nihilego may be inspired by a jellyfish, especially Aurelia aurita. It may also draw inspiration from Myxozoa, a class of microscopic Cnidarian parasites, particularly the Myxobolus cerebralis that are known to infect "whirling disease" in various fish species. The disease causes the fish to develop bone and neurological abnormalities that cause them to swim in a corkscrew-like "whirling" motion. It could also draw some design from Lillie due to its "sunhat" being similar to Lillie's.
The merged form of Lusamine and Nihilego may be based on Turritopsis dohrnii, a species of jellyfish that is biologically immortal and can revert to a young, immature state if it is too old or stressed.
Nihilego may be a combination of nihil (Latin for indefinite nothing) and ego (Latin for I), which together roughly translates to "I am nothing" or "I have nothing"; or ego. In either case, it serves as a reflection of Nihilego's ability to enslave its hosts.
Utsuroido may be a combination of 虚ろ utsuro (void), void, and id. It also might refer to 鬱 utsu (depression), 移ろい utsuroi (the act of moving from host to host), and android or the suffix -oid (meaning "of similar form").
In other languages
From 虚ろ utsuro, void, id, and possibly the suffix -oid (meaning "of similar form")