What is that thing?

oh, you don't play terraria?

I have, just never saw that thing.

which probably means you haven't tried the last update.

Plantera. Giant flower beast that lives in the Underground Jungle in hardmode. Rather nasty and arguably the most difficult single boss in the game.
Pictured here is its RAGE MODE; It flips out and murders your face off if you try to drag it out of the Underground Jungle. (Originally, you could just lure it to the surface, which crippled its mobility and made it way easier to dodge its attacks, which made the fight into a joke. Now... not so much.)

Thanks.