cards that specifically state that they cannot be special summoned from the graveyard, like ancient gear golem, or the very limited card: Tyler the great warrior (only one in the world) also an elemental hero fusion
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Lava Golem's own special summon effect can only special summon itself to the opponent's side of the field. Like other 'can only be special summoned by..' monsters, if you summon it correctly, and it is then destroyed, it can be brought back to the field by cards like Monster Reborn. In Lava Golem's case, it will be special summoned to your side of the field if you do this.
Basically, when you special summon a monster with 1500 or less ATK with cards like marauding captain, call of the haunted, or monster reborn, you can activate this card when your opponent has a face-up monster to special summon all copies of that monster. Your opponent can special summon all copies of his monster too, but that's a rare chance IMO.
Rainbow Neos 'cannot' be special summoned except by Fusion Summon of the listed components. There is no way around this, not even cards like Metamorphosis or Cyber Stein.
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Only if the card specifically says it can summon a Ritual Monster, like Ritual Foregone. Otherwise, you can't do it, cards like A Hero Emerges, or Monster Gate, cannot special summon a Ritual Monster.Also if you summon the Ritual Monster properly, by Ritual Summon, and it is then sent to the graveyard for whatever reason, then you may bring it back to the field using cards like Monster Reborn. However you cannot simply discard a Ritual Monster from hand and try using Monster Reborn straight away, they are a form of special summon only monster, simply by being Ritual Monsters.