Pheromosa is an Ultra Beast that resembles a feminine anthropomorphic 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 reach speeds exceeding 120 mph (200 km/h), which no other known life form has previously been able to reach. 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.
Due to Beast Balls having a 5× catch rate modifier against Ultra Beasts, this means that they can never fail to catch a Pheromosa in Sun and Moon.
Pheromosa was the inspiration for the naming of Nocticola pheromosa, a species of cockroach first discovered in Singapore and described in 2023 by entomologists Foo Maosheng and Cristian C. Lucañas. According to Foo, he and his collaborator are both Pokémon fans, and that the cockroach was named after Pheromosa due to both having similar features, such as long antennae, hood-like wings, and long slender legs.
Pheromosa may be based on an American cockroach that has molted, which causes it to be white in color, and may also draw inspiration from a copepod. It also resembles a fashion model, given its idle pose, and its body resembling an artful set of clothes. Pheromosa's overall design and personality seems to contrast the real world perception of cockroaches, which tend to be considered creepy and unclean.
Pheromosa may be a combination of pheromone and hermosa (Spanish for beautiful) or formosa (Italian for shapely; Portuguese for beautiful).
Pheroache may be a combination of pheromone, feroce (Italian for fierce), and cockroach.