Seven Great Youma
The Seven Great Youma were introduced in episode 25 when Queen Metalia revealed that the Silver Crystal had encased them after the battle on the moon. After they were trapped, the crystal shattered into seven colored pieces called Rainbow Crystals, each holding a Youma; the entrapped Youma disappeared on Earth, and were reincarnated inside normal humans. Queen Beryl charged Zoisite with locating and extracting each crystal piece, using a special homing crystal which had previously belonged to Nephrite, the Black Crystal.
The search for the crystals lasted seven episodes, and each battle for a crystal piece normally began when Zoisite, sensing the crystal piece, would appear in front of the piece's holder and use the Black Crystal to extract the crystal right from the carrier's heart. The extraction tended to be slow and painful for the victim, and when the crystal was finally extracted the person reverted to their Great Youma form, attacking the Sailor Senshi who came to stop Zoisite from getting the Rainbow Crystal. Once the Youma was weakened, Sailor Moon would not kill it, but would instead use Moon Healing Escalation on the person to re-seal the Youma inside them.
Later, in episode 41, Queen Beryl sent Prince Endymion with the Black Crystal to recapture the seven holders, in order to combine them into one invincible super-Youma the Sailor Senshi would have been incapable of defeating. Endymion was able to capture six of them, only lacking Rei's Grandfather by the time Sailor Mercury fought him and destroyed the crystal, thus thwarting the Dark Kingdom.
In the first English dub, they were collectively called the "Seven Shadows," and their combined form was called the "One Invincible Shadow."
List of the Seven Great Youma
The following is the list of the Seven Great Youma, as well as their human forms, their crystal color, and the party that eventually won the Rainbow Crystal prior to episode 34:
Of these, only two of the characters - Reika and Rei's Grandfather - made any major appearance in episodes beyond the Seven Great Youma plot. These two also were characters which were created by Naoko Takeuchi, and thus appeared in the manga as minor characters. The rest appeared only in the anime, and only in the first season.
All the Rainbow Crystal carriers appeared briefly in episode 41, in which Ryo specifically was central to the episode. In episode 45, the DD Girls created an illusion of Ryo Urawa, and in episode 54 of the English dub he was mentioned by name. Other than that, though, none of them except the two listed above were mentioned again.