Hybrid Monsters are colored dark green, (not to be confused with Spell Cards. But like Fusion Monsters and Synchro Monsters, they may only be included in the Extra Deck, and cannot exist in the hand or Main Deck. If a Hybrid Monster is returned to it's owner's hand or Deck, it is instead sent back to the Extra Deck.
A Hybrid Monster is a card that consists of 1 monster and 1 or more Spell/ Trap cards. A Hybrid can only be Summoned by having the required Hybrid Material Monsters in your hand and then activating "Molecular Fusion." Hybrids will always have an effect, usually a combination of the monsters and other Hybrid Material cards' effects. Hybrids are immune to the Spell Cards/Trap Cards Card that is the other Hybrid Material card. Hybrids were first introduced in the FTCG Booster Pack, DNA Experimentation.
If a Hybrid Monster is Hybrid Summoned to the field, and then sent to the Graveyard, it cannot be Special Summoned to the field from the Graveyard with cards such as Premature Burial, Call of the Haunted, or Monster Reborn. In order for a Hybrid Monster to be returned from the Graveyard to the field, you must use the Hybrid Summoner.