Negamonsters are the monsters of the week sent by the Negaverse. Most of them are destroyed by Sailor Moon's Moon Tiara Magic, and some of them by attack combinations. The others are destroyed by other Sailor Scouts. Some of them are people turned into monsters, that Sailor Moon heals with Moon Healing Activation, which permanently kills the monster they were turned into, preventing the Negaverse from turning them into a monster again.
Neflite's monsters are implanted into items close to a person, whose energy will soon reach its highest point. When it does, the monster will appear and drain their energy, until it is destroyed.
Leo the Lion and Soul Shadow are the first male Negamonsters, and interestingly enough, are not sent to absorb energy, but rather to kill Sailor Moon and to find the Moon Princess (respectively). Leo the Lion is not killed in the episode he appeared in, and survives at least until the Negaverse is destroyed with Queen Beryl.
Though Zoycite does have regular monsters working for her, during her arc she turns people into monsters, where they must be healed by Sailor Moon.
These are the most powerful Negamonsters, sealed inside the crystal carriers. When Zoycite turns a crystal carrier into a monster, she is awakening the Shadow within them. When the person is healed by Sailor Moon, the Shadow is killed and cannot be awakened again, as later confirmed by Malachite in Tuxedo Unmasked.
Unlike most Negamonsters, all of them (except Veena and Techniclon) are male.
Besides Seven Shadows, Zoycite can turn other people into monsters as well.
Malachite's monsters are all people turned into monsters, which must be healed by Sailor Moon. Their task is to find Sailor Moon and take the Imperium Silver Crystal. Though they also aim to kill her, Prince Darien (brainwashed by the Negaverse) is against the idea, and for that reason sometimes helps her fight them. These can turn back into their original forms at will, but still have the minds of Negamonsters, until Sailor Moon heals them.
Prince Darien also awakened a sea monster called Ancient Creature of the Lake, though she is not technically a Negamonster, but rather a girl who got jealous and became a monster.