Plantera

Plantera is a hardmode boss introduced in 1.2. It is summoned by destroying a pink plant that grows in the open areas of the Underground Jungle. Said plant only spawns after another hardmode boss has been defeated first. Upon death, Plantera drops a number of items, including a key that unlocks the underground jungle dungeon. When it is defeated for the first time it will always drop a Grenade Launcher with Rocket Is. Plantera changes to its second for when it reaches half health, which is 15000.

Trivia

 * "Plantera" is possibly a parody of the American groove metal band Pantera.
 * Plantera also drops "The Axe," which looks like an electric guitar.
 * "Plantera" is the Swedish word for Plant (as in "to plant").
 * It is possible that the name "Plantera" is a portmanteau of "Plant" and "Terror."
 * Plantera could possibly be a reference to the giant plant from Little Shop of Horrors