Small crate

The Mystery Box are objects found in Metroid: Other M. They usually have items in them like the Missile Expansion where Samus has to use either bombs or missiles to break them open. The area where Samus fights the RB176 Ferrocrusher is where you find many empty Mystery Boxes all but one that has a Missile Expansion in it.

Trivia

 * The Mystery Boxes are sort of similar to the Unkown Items found in Metroid: Zero Mission except that unlike the Unknown Items that contain Suit Upgrades and cannot be used until Samus defeats the Ruins Test, the Mystery Boxes contain expansions instead and they're usable as soon as Samus obtains them.