Ariados is a red Pokémon that resembles a spider. It has purple eyes, a pair of white mandibles, and a white horn on its forehead. Around its abdomen are two black stripes and there are two black spots and a yellow spinneret on its rear. The spots and spinneret together form a pattern similar to a face. Ariados can produce thread from both its spinneret and its mouth. It has four yellow legs with two purple bands each; on its back are yellow structures similar to its legs with a single purple stripe each. Its feet are tipped with tiny hooks that allow it to climb ceilings and vertical walls.
Ariados makes its nest in the depths of temperate and tropical forests and leaves after dark to hunt prey. While Ariados wanders in search of prey, it spins a single thread from its rear that leads back to its nest. When prey is captured, it is wrapped in thread and then attacked by fangs. Ariados then drinks the victim's bodily fluids at leisure. Sometimes, it will instead release prey with only a single thread attached. The thread is then followed to both the caught prey and its friends. Ariados and its pre-evolution Spinarak are the only known Pokémon that can learn Toxic Thread. In the past, this was also true of Spider Web.
In Following A Maiden's Voyage!, multiple Ariados attacked Dawn and her Piplup after the latter got caught in one of their webs. However, they worked together to escape them, and the Ariados were sent flying by Piplup's Bide.
In Stopped in the Name of Love!, eight Ariados attacked Dawn and her Piplup, mimicking the first day of their journey. Just like the first time, they were sent flying by Piplup's Bide.
In DPS01, multiple Ariados led by a Shiny one attacked Dawn and Shinko, once again mimicking the attack at the start of Dawn's journey. This time, they were defeated when Dawn's Cyndaquil evolved into a Quilava and used Eruption.
Ariados may be named after Ariadne, a Cretan princess in Greek mythology associated with strings and mazes. In the myth, Ariadne gave Theseus a ball of twine before he entered the Labyrinth to slay the Minotaur, so that he may trail the strand behind him and find his route back to her. Ariados may also be a combination of arachnid (Latin for spider) and 脅す odosu (to threaten) or dos (Spanish for two, used possibly because it has two stingers, or because it is the second form in its evolutionary line).