It is not known to evolve into or from any other Pokémon.
It is one of the Ultra Beasts and is known by the code name UB-01 Symbiont (Japanese: ＵＢ０１ Ｐａｒａｓｉｔｅ).
Nihilego is an Ultra Beast that resembles a jellyfish. The bell is semi-translucent and encircled with several white, star-shaped markings. The wavy rim of the bell flares outward and is bright blue. Its tentacles appear gelatinous and give it a vaguely humanoid shape. Several tentacles mesh together forming the appearance of cape or long hair, while four shorter tentacles resemble shorter hair and arms. Two long tentacles make up the legs of the human-shape. In addition to its shape, it is sometimes seen acting like a young girl.
Nihilego is a parasitic organism and, by latching onto one's head, is 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 Wicke, the Aether Foundation's research it seems to act mainly on a self-preservation instinct.
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 and have numerous deep black spots with various colored diamond-shapes inside. Four of the tentacles are connected to the bell 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 larger spine. While the anime shows that Nihilego can willingly end this merger and it may be disrupted in other ways as well, the lingering toxins and the shock of the separation can leave the host in critical condition, both physically and psychologically.
Nihilego debuted in a flashback in A Masked Warning!, where Gladion revealed to Ash that Lillie was once attacked by one when she was younger, resulting in her present fear of touching Pokémon. It appeared again in a flashback in Mission: Total Recall!.
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 cause the infectious "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. Visually, and due to its status as an Ultra Beast, it may derive inspiration from Phyllorhiza punctata, an invasive species in the waters surrounding Hawaii.
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". Ego may also refer to the part of the psychic apparatus that governs rational decision-making, and together with nihil reflects how the creature may have no ego, and only have an id (the survival and instinctual part of the psyche). The former may reflect its victims' lack of will outside their focus on Nihilego, while the latter may represent Nihilego's role in dominating the desires of its victims.
Uturoid 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")