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. The first time he dies he always drops a Grenade Launcher with Rocket Is.

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