Youma is the generic term for a 'monster of the day' created by the Dark Kingdom. None of them from the television series were given names, nor in official publications, like in other Tokusatsu. Youma, loosely, translates to bewitching demon. There was no consistent form they were summoned, most of them just appeared, some were created from stones.
This translucent youma possessed Naru's mother in order to steal energy, which led Usagi to transform into Sailor Moon for the first time. She used her 'Moon Healing Escalation' from her Moonlight Stick to remove the youma from her friend's mother.
ACTS 2 & 20
Jadeite transforms a statue of a human and dog into two Youma. They drain energy from students in a cram school Ami was attending. The humonoid Youma pretends to be Ami's teacher to steal her energy. Sailor destroys one youma with her Moon Twilight Flash and the other is destroyed by Sailor Mercury's Mercury Aqua Mist. The humanoid Youma appears again in Act 20 to tease and distract Rei.
ACTS 3 & 20
This propeller-themed Youma pulled girls through a door to another dimension to drain their energy. It concerntrated on wind attacks. Rei and Usagi are pulled into the dimension. Sailor Mars destroys the Youma using her Youma Taisan attack. He appears again in Act 20 to tease and distract Rei.
ACTS 4 & 20
Nephrite turns a plant into a Youma to possess a Jewel owner at a party. Sailor Moon uses Youma Taisan, but the Youma splits into two. One attacks her while the other runs after Mercury. Once Sailor Moon transform, the two Youma split split into a third one. Sailor Moon, Mercury, and Mars use their three attacks to destroy the three Youma. The yellow one appears in Act 20 to tease and distract Rei.
ACTS 5 & 20
Nephrite tells this Youma to search for the Silver Crystal and snatches a pendant from a woman, thinking it is the Silver Crystal. It bounces around quickly place to place, hence hard to catch. She also says bouncing sounds. Sailor Moon and Sailor Mars use a combine force of Moon Twilight Flash and Youma Taisan to destroy it. She appears again in Act 20 and teases Sailor Moon.
This youma takes over the body of Takeru, a popular boy among the girls. It makes statues out of his female victims and Jadeite keeps them in a dimension to steal their energy. It had the ability to wrap its snake-like whip around people's necks. It had the ability to put a spell over women. Sailor Jupiter destroys it with her Supreme Thunder. It seems to be inspired by Gorgons.
ACTS 7 & 20
This ninja-like straw man first fought Sailor Moon at night. He leaves a decoy for others to hit. He possess Takai, Motoki's friend and Rei's date on a triple date to steal Rei's energy. Remembering her previous fight with this Youma, Sailor Moon waits until Jupiter has destroyed the decoy and then turns and sends her attack at the real Youma to destroy it. He appears in Act 20 to tease Sailor Moon.
ACTS 10 & 20
The papers of Zoisite's Requiem swirl together to form a Youma. She sings an accompaniment to his music. Youma's song grows stronger the Princess of Estoa faints. The Youma attacks by singing multicolored musical notes. Sailor Moon destroys it with a combination of her Moonlight Stick and her crystal's energy. This Youma appears again in Act 20 and teases Sailor Moon.
ACT 12
This Youma possesses Minako's manager Sugao Saitou, in aims to find the Silver Crystal. It can wrap vines from its hands. It binds Minako and she breaks free from him. The Youma separates from Saitou's body and Sailor Moon tries to use her Moon Tiara Boomerang, but the Youma just knocks it aside. The Youma manages to trap the three Senshi. Sailor Moon destroys the Youma with her Moon Twilight Flash.
ACT 13
A schoolgirl named Ayumi is slowly turned into a Ying-Yang Youma by Kunzite. Sailor Moon is reluctant to attack because she doesn't want to harm the innocent person inside. Luna tells her to believe in her power, because she has no choice. Sailor Moon apologizes to the Youma and uses Moon Healing Escalation to turn Ayumi back to normal. Kunzite blows on the hair and it wraps around Sailor Moon's neck that might have turned her into a similar Youma but Usagi's power was too strong.