A loose, sleeveless outer garment worn over other clothes, often associated with historical or fantasy contexts. The French loanword マント is more specific than the general English 'cloak' and is commonly used in Japanese for capes in anime, games, and period pieces.
from French manteau
See also: マンテル
The wizard was wearing a black cloak.
The wizard wore a long mantle.
That hero's trademark is a red cape.
He threw a cape over his shoulders.
He took off his cloak upon entering the room.
The cloak billowed wildly in the wind.
マント (cloak) and マントル (the Earth's mantle, or a fireplace mantel) sound almost identical but refer to completely different things.
Borrowed from French 'manteau', meaning a cloak or mantle. The form マント reflects the French pronunciation adapted to Japanese phonology.