Bonetail

Bonetail is a large zombified Dragon in Paper Mario: The Thousand-Year Door, living at the bottom of the Pit of 100 Trials.

He is the oldest member of the "tail" family, older than both Gloomtail and Hooktail. As his name implies, he is merely a skeleton of a Dragon, implying that he has been living far longer than the other members of the family. The reason why he became a skeleton is that he never ate anything for so long and died of old age. Any actions outside of the Pit of 100 Trials, if any, are unknown.

He was once a pet of the Shadow Queen and was put in the pit to guard and punish anyone she wanted.

After Mario and his teammates reach the bottom floor of the Pit of 100 Trials and defeat him, they'll earn the Return Postage Badge.

Bonetail is the strongest enemy in the game, even stronger than the Shadow Queen (partially because when Mario gets through the Pit of 100 Trails he'll be exhausted from the many previous battles).

Bonetail's breath can confuse and cause sleep. He can also restore his HP if it's low.

After being defeated, he burps up an old chest containing the Return Postage badge.