Pheromosa is an Ultra Beast that resembles an anthropomorphic, feminine cockroach. It is primarily white with a thin build. Its torso is composed of five thin discs that droop slightly downward. The widest disc makes up its shoulders, the second widest its hips, and the two smallest its chest and waist. There is a gold ovoid inside of the widest disc, where its long, thin neck protrudes from. Its arms and legs are also long and thin with gold joints. Its hands and feet are both composed of two golden claws. Pheromosa has a small, semi-flattened head with a yellow, crown-like structure on top. The two tallest points of the crown point to its long, thin antennae, which are white with small gold balls at the tips. Attached to the back of its head are two structures resembling insect wings. The wings are transparent, curve slightly inward, and reach nearly to the ground. It has narrow, purple eyes with bright blue pupils.
According to Wicke from the Aether Foundation, Pheromosa's lithe, elegant body is capable of sudden feats of acceleration, able to twitch its limbs at an estimated 120 mph (200 km/h) instantaneously, a speed previously unseen in the world. It also produces a sort of disarming pheromone, which was described as "previously unknown to science" that causes those nearby to become awestruck by its presence. It displays a considerable reluctance towards touching anything as if it interprets the Pokémon world as being unclean somehow.