The Zapinator is a 3DS-exclusive,Hardmodemagic weapon. It has an approximate 1.25*1/80 (1.25%) chance of being dropped by Plantera, but the exact drop chance is unknown. It shoots green projectiles very rapidly. It can be considered as an upgrade to the Space Gun.
Its best modifier is either Masterful for mana conservation or Godly/Demonic for higher damage output. It cannot get the Mythical modifier due to its low mana cost.
Because the Zapinator consumes lots of mana when fired rapidly, the Mana Flower is helpful to continuously use it.
The Zapinator was used by the developers in the Desktop version to test certain content prior to Terraria's official release, used to kill enemies instantly so they would not be interrupted during testing. It was originally released on the Desktop version, but was removed in the 1.0.1 update, replacing them with Angel Statues. At the time, it was essentially the most powerful weapon in the game, though since update 1.1 and the release of Hardmode, this has ceased to be true.
This could explain why the Zapinator was removed from the Desktop version, as it could cause copyright problems between Re-Logic and Nintendo.
This might also explain why it was subsequently re-added to the Nintendo 3DS version – there would be no conflict with Nintendo for this instance.
Despite that, both the Nintendo Switch version and the Wii U version of Old-gen console version lack the Zapinator. While the former may receive the Zapinator at some point in the future, the Wii U never will, as the Old-gen consoles are no longer receiving updates. However, due to the introduction of the Gray Zapinator and the Orange Zapinator, it is highly unlikely that this will be introduced into the Nintendo Switch version.
The projectile shot by the Zapinator is also used by the Space Gun.