Bonetail is a large, skeletal dragon in Paper Mario: The Thousand-Year Door residing at the bottom of the Pit of 100 Trials. There is some regional dispute over the gender of Bonetail, as while the English version depicts Bonetail as male, other versions depict Bonetail as a female. In the Italian version, Bonetail's name is derived from "Cassandra", a girl's name.
It is the older sibling of Gloomtail and Hooktail. Unlike its still-living siblings, it is an undead dragon, having died of old age after not eating anything for centuries at the bottom of the Pit of 100 Trials. It was once a pet of the Shadow Queen, but it is never stated why or how Bonetail came to reside at the bottom of the Pit. According to Grifty, Bonetail, along with its two siblings, went around the world terrorizing people some time before the Shadow Queen was imprisoned behind the Thousand-Year Door.
After Mario and his teammates reach the bottom floor of the Pit of 100 Trials, Bonetail will roar, making it questionable if it can talk, as it never demonstrates that ability. Bonetail's many breaths cause a variety of status ailments in addition to dealing damage. Its blue breath can freeze Mario, green breath can cause confusion, purple can shrink Mario or his partner, and white has the effect of a Sleepy Sheep. All of its fire breath attacks can be avoided with Shell Shield or simply guarding. If Mario defeats Bonetail, it coughs up a chest containing the Return Postage Badge.
Bonetail is much stronger than its younger siblings, Hooktail and Gloomtail, and even than the Shadow Queen. It is the strongest boss in the game; Bonetail can restore 20 HP when having lower HP, and has the most HP out of any enemy. Additionally, Bonetail has the same default attack and defense stats as Gloomtail, with other buffs added throughout the fight (but note that Bonetail doesn't have all of Gloomtail's attacks, like Earthquake or Megabreath).
- Bonetail, along with Gloomtail, possesses one of the very few attacks in the game that cannot be blocked with a Guard or Superguard: the bite attack. Other such attacks are the Bulky Bob-ombs and Bob-ulk's explosions.
- If the player is skilled enough, it is very possible to defeat Bonetail before fighting its youngest sibling, Hooktail. By using a series of glitches, Bonetail can be reached before beating the prologue.
- Despite being the strongest boss, Bonetail only offers one Star Point after defeat, just like the Shadow Queen. This is often considered a gag.
- In the early version of Bonetail's head, it depicted it having blue eyes.