A Legend Monster (Legenddo Monsuta) is a type of monster made by mrd256 that is Special Summoned from the Extra Deck by a Legend Summon by placing monsters from your field into your Legend Gauge. Their frame is a dark red color with a wave-like pattern.
How they work
Legend monsters can activate their effects by paying (sending to the Graveyard) cards from the Legend Gauge. Because of this, they are very similar to Xyz Monsters, except for one thing. While Xyz Monsters are limited to what Materials are attached to them, any Legend Monsters can pay any card from the Legend Gauge, even if it wasn't used for their summon. So the more Legend Summons you perform, the more Gauge you can build up to use for effects.
They are called Legend Monsters as they are monsters of old Legend being called forth from times long forgotten to fight in the present.