- "Whaaaaaaaaaaaaaaaat!? You destroyed my precious younger sister!? I wondered why she hasn't written to me so recently..."
- ― Gloomtail expresses his surprise that Hooktail had died.
Gloomtail is in the middle-aged member of the Tail family being the younger brother of Bonetail and the older brother of Hooktail. Gloomtail desires to destroy Mario after hearing that he defeated his younger sister, Hooktail. He guards the Star Key required to unlock pedestals in the Riddle Tower.
When Mario and co. reach the end of Palace of Shadow, the find Gloomtail who desires to battle Mario and co. Gloomtail has attacks that are dangerous, especially the Megabreath move being his most powerful attack. He also attacks with his poison breath, doing an Earthquake which, however, doesn't hurt Vivian and Flurrie. He also can finally chomp or stomp on Mario and co.
What an Enormous Dragon!
Dragon Slaying Battle
- Gloomtail is the only dragon boss in Paper Mario: The Thousand-Year Door that does not have a Super Paper Mario equivalent.
- The room Sir Grodus is fought in is directly below Gloomtail's room, which means that Bowser and Kammy were in Gloomtail's room when they fell through the floor.
- If Mario comes in the fight with a partner that wasn't present in the Hooktail fight, they will mistake Gloomtail for Hooktail and go over some descriptions Mario probably told them offscreen.
- Bonetail has the same default attack and defense stats as Gloomtail does.
- Gloomtail's room is similar to the spike room in Hooktail's Castle if you use Bobbery at the right end at the wall, a room will open and an Ultra Shroom and a Jammin' Jelly can be collected.
- Gloomtail has the third strongest enemy attack in the entire game. His Megabreath can do 16 points of damage, 2 points higher than the Shadow Queen's Megawave attack. However, if the Shadow Queen charges up (Increases attack and defense) and then charges up for the Megawave, her attack does 17 points of damage. The strongest enemy attack belongs to the Amazy Dayzee, with a power of 20.
- Gloomtail claims that Hooktail would send him letters but it's unknown how Gloomtail got the letter before Mario broke the seal on the Thousand-Year Door.