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.

Trivia

 * "Plantera" is possibly a parody of the American thrash metal band Pantera.
 * Plantera also drops "the Axe," which looks like an electric guitar.
 * "Plantera" is the Swedish word for Plant.