Adrian Gecko with "Forbidden Beast Nibunu" and "Forbidden Beast Inun" alongside "Forbidden Mantra".
"Forbidden Beast" known as "Sealed Beast" (ふういんじゅう Fūinjū) in the Japanese version, is an archetype used by Adrian Gecko in the Yu-Gi-Oh! GX manga. Many of them resemble armless reptiles with effects linked to the "Forbidden Mantra", which holds their power.
They were initially called "Sealed Beasts" in the manga, matching their Japanese name, but their name was later change to "Forbidden Beasts". This change seems to have been inspired by that of "Exodia", which Adrian's anime counterpart used.
The "Forbidden Beasts" effects require the "Forbidden Mantra" in play, otherwise they are nearly powerless. The only exceptions to this are "Forbidden Beast Inun" and "Forbidden Beast Nibunu". When "Forbidden Mantra" is in play, their deadly effects of destroying cards come into effect. Some of them also focus on Summoning from the Graveyard and getting "Forbidden Mantra" onto the field.