Wondrous item, rare
This square rosewood cage measures 6 inches per side and has an ornate brass handle attached to the top. You can use an action to touch a beast of size large or smaller with the cage. The creature must make a successful DC 15 Constitution saving throw or be drawn into the cage in miniature form. If you touch another creature with the cage while it is occupied, nothing happens.
Once in the cage, the beast requires food, water, and air as normal. Opening the cage door causes the beast to return to normal size in the nearest unoccupied space, the cage is broken and unusable after this.
-Mordenkainens Tome of Marvelous Magic I (modified)