It was the one who planted the giant flower covering his subterranean burrow in the top of a hill.
Twilight and Princess Cadance climb to the top of the hill and discover that the flower Discord mentioned is towering in size. When they pull it from the ground, the Tatzlwurm emerges from underground and angrily attacks them, claiming his flower, and forcing the princesses to fight.
First the Tatzlwurm tries to eat the princesses with strong bites, but it fails, them the princesses ftry to escape flying, but the Tatzlwurm jumps in an attempt to reach and then eat them. It succeeded in trapping Cadence with his tentacles, but Twilight saves her by blasting the Tatzlwurm with magic. Cadence escapes flying but now the Tatzlwurm attacks Twilight in the same way (only without jumping), and Cadence saves her by shooting the monster in the same way 3 times. Now the Tatzlwurm jumps trying to eat both at once, almost succeeds, if it was not because he was attacked from inside. The princesses blast the creature with their alicorn magic, drive it back underground, and seal the hole it escaped from with rocks. Twilight and Cadance return to Discord with the flower and find him perfectly healthy and dancing. Twilight becomes infuriated over the fact Discord faked being sick, but Discord explains that he thought Twilight wasn't really his friend—she always says she is but has never shown it.
He tells Twilight his faking illness was a test of friendship, and the fact that she would travel to the ends of Equestria for his sake means she passed. Discord wonders if his "test" put a damper on Twilight and Cadance's quality time. Cadance says it didn't, stating that life in the Crystal Empire has become predictable and that she enjoys a little excitement now and then.
Suddenly, the Tatzlwurm emerges from underground again and sneezes on the three. While the two princesses are protected by Cadance's bubble, the unprotected Discord turns green.
Friendship is Magic