The Monster Locker is a building that allows the player to research and unlock new monsters. Each monster has a varying amount of Putty required to unlock. The Monster Locker must be built during the tutorial. It reaches a max level of four. Each level unlocks more monsters. The finished build unlocks four monsters: Pokey, Octo-ooze, Bolt, and Fink . The first upgrade (Lvl 2) unlocks: Eye-ra, Ichi, Bandito, Fang, and Vorg . The second upgrade (lvl 3) unlocks: Brain, Crabatron, Project X, Slimeattikus, and Zafreeti . The final upgrade (lvl 4) unlocks: Teratorn, Wormzer, and D.A.V.E. If the Monster Lockeris destroyed while it is unlocking, it will delay the unlock time, and continue when it is repaired.
Its Inferno counterpart is the Strongbox, as they both can unlock monsters. The only difference is the size and shape, Strongbox is smaller and circular and the Monster locker is taller and of an oblong shape.
The speed up to complete unlocking a monster says training instead of unlock e.g.complete brain's TRAINING now.
In the beta stage of Backyard Monsters, players were able to build two Monster Lockers. Don't be surprised if you see two Lockers in one yard, fortunately they can only unlock one monster at a time. They're still given the advantage of having a longer NEC, though.
You can recycle the monster locker when it is unlocking but you will cancel the unlocking of what you are unlocking.