"Monsters dwell in every heart. We only think civilization tames them."
—Euphemia, Nexus watcher

Beast Within

Destroy target permanent. Its controller creates a 3/3 green Beast creature token.

Beasties tend to be shy, kind to humans, and absolutely ruthless toward Valgavoth's monstrous spawn.

Duskmourn: House of Horror Commander

Bloomburrow Commander

Modern Horizons 3 Commander

The Lost Caverns of Ixalan Commander

"Do you recognize them? The Chagaskas. Your worst fears, my creativity."
—Zellin

Doctor Who

If the permanent is still a legal target but is not destroyed (perhaps because it regenerated or has indestructible), its controller still gets the Beast token.

If the target permanent is an illegal target by the time Beast Within tries to resolve, the spell won't resolve. No player creates a Beast token. If the target is legal but not destroyed (most likely because it has indestructible), its controller does create a Beast token.