Special Summon-only monsters, often referred to simply as Semi-Nomi monsters, are monsters with the effect "Cannot be Normal Summoned or Set. Must first be Special Summoned..." (before the Problem-Solving Card Text update, the text read "[This card cannot be Normal Summoned or Set.] This card can only be Special Summoned...").
A Special Summon-only monster must be Special Summoned by its listed effect before it can be Special Summoned by another effect, as opposed to "Nomi" monsters, which can only be summoned by their own effect, even after they have been properly Special Summoned. "Aqua Spirit", "Harpie Lady Sisters" and "Black Luster Soldier - Envoy of the Beginning" are all Special Summon only monsters.
After a Special Summon-only monster has been Special Summoned properly (meaning its summon was not negated by cards such as "Solemn Judgment" or "Solemn Warning", "Black Horn of Heaven and etc...), it can be Special Summoned through other effects. This only lasts as long as the monster does not return to the hand or Deck; as long as the monster remains on the field, in the Graveyard, or is banished, it can be Special Summoned again with other card effects.
- A player removes from play three Fiend-type monsters to Special Summon "Dark Necrofear". If it is destroyed and sent to the Graveyard, it can be revived with cards such as "Monster Reborn".
- If, afterwards, it is removed from play by "Dark Core", you can still Special Summon it with cards such as "Dimensional Inversion".
- If after all this, "Dark Necrofear" is returned to the Deck or hand (such as by "Phoenix Wing Wind Blast" or "Compulsory Evacuation Device"), then you must remove three Fiends to Special Summon "Dark Necrofear"'s again. You cannot Special Summon it from the hand or deck by other cards, such as "Monster Gate" or "Doom Shaman".
- If it is later removed from play or sent to the Graveyard (such as by "Graceful Charity"), it still cannot be Special Summoned.
Note that Fusion, Ritual, Synchro, and Xyz Monsters are treated as Special Summon-only monsters, unless stated otherwise. Also unless stated otherwise, Fusion Monsters are properly Special Summoned by a Fusion Summon, Ritual Monsters by a Ritual Summon, Synchro Monsters by a Synchro Summon, and Xyz Monsters by an Xyz Summon.