Pendulum Monster is the new type of Monster Card introduced in Yu-Gi-Oh! ARC-V. They act as either Monster Cards or Spell Cards (hence, their color coding being half orange (yellow if they have no Monster Effect) and half green), and can be placed in the new Pendulum Zone. These monsters can be used to perform Pendulum Summons, a new mechanic that allows a player to perform high-level summons.
Pendulum Monsters, while still treated as Effect Monsters with Levels on the field, use a Pendulum Scale to conduct Pendulum Summons. A Pendulum Monster is first placed in the Pendulum Zone as a Spell Card, followed by another Pendulum Monster in the second Zone, where it then uses its Pendulum Scale to determine the Levels of monsters than can be Pendulum Summoned.
When a Pendulum Monster would be sent from the field to the Graveyard, either as a Monster Card or Spell Card, it is placed face-up in the Extra Deck Zone instead. If a Pendulum Monster is sent from the hand or Deck to the Graveyard, or if they are detached, they go into the Graveyard like normal. A Pendulum Monster in the Extra Deck Zone can be Summoned once again through Pendulum Summoning, so long as its Level is within the Scales.
While in the Pendulum Zone, they are treated as Spells, and apply their Pendulum Effects. (Their effects as monsters are not applied.) You cannot Set a Pendulum Monster into the Pendulum Zone. However, although they are treated as Spells, they are special cards that are not treated as a Normal, Continuous, Field, Equip, Quick-Play, or Ritual Spell Card. Pendulum Monsters placed in your Pendulum Zones do not count towards you 5 card total in your Spell & Trap Card Zone. You cannot replace Pendulum Monsters that are in your Pendulum Zones like you can with Field Spell Cards. (They remain in the Zone as long as they are not destroyed.) As long as your Pendulum Zones are open, there is no limit to how many times you can place Pendulum Monsters into your Pendulum Zones per turn.
As the Pendulum Effect and the Pendulum Scales take up the space where the Set Number would usually be placed, this value is placed in the bottom left corner of the card instead, in the same line as the ATK/DEF values of the Pendulum Monster.