It is one of the Ultra Beasts and is known by the code name UB Stinger (Japanese: ＵＢ：ＳＴＩＮＧＥＲ UB: Stinger).
Naganadel is an Ultra Beast that has both draconic and insectoid features. Its upper body is slender with thin arms and a long neck. The upper chest, neck, head, and arms are light purple, while its waist is dark purple with a fuchsia stripe on each side. Each shoulder has a long gray spike and each hand has three pinkish purple claws. Running down each side of its neck is a dark purple stripe, which connect and form a loop on its chest. The lower area of the loop is fuchsia and has a short, downward extension in the middle. At the end of its long neck is its head, which has a long, spiked crest on the top. On the sides of its mouth are a pair of curved, gray spikes that act as mandibles. Its glowing cyan eyes are conjoined, rimmed with fuchsia, and have a spiky crest on either side, which gives them a visor-like appearance. A pair of light purple wings sprout from the center of its back. These wings have a single fuchsia claw at the joint, two spikes along the wing-arm, and darker purple membranes with fuchsia markings near the spikes.
The lower half of Naganadel's body houses its brain. It resembles a swollen wasp's abdomen and is filled with poisonous liquid. It is covered in stripes of light and dark purple and has three hollow stingers at the tip. The stingers are gray and have a fuchsia marking around them. The largest stinger is in the center; the other two are considerably smaller and sit on either side. These stingers are connected to its brain, so it will react violently if they are touched. Its main stinger can eject a luminescent, adhesive poison over a distance greater than six miles. In the anime, Naganadel is shown to be able to communicate with humans through telepathy and show them visions of the past.
Naganadel has the lowest base stat total of all fully-evolved Ultra Beasts, having a total of 540 while the rest have 570.
Naganadel may be based on a Western dragon or wyvern, a kind of dragon that possesses two arms and wings. Its abdomen resembles that of a wasp. Specifically, it could be based on the tarantula hawk or Ichneumonidae family of wasps. Its Shiny coloration may reference wasps such as the yellowjacket. Like its pre-evolution, its needle-like appendages resemble the needle of a syringe, with the abdomen representing the barrel or tube and the venom representing the liquid inside.
Naganadel is a combination of Nāga and Nadel (German for needle).
Agoyon may be a combination of 顎 ago (jaw), ago (Italian for needle), 四 yon (four), and dragon or 용 yong (Korean for dragon).