In How to Train Your Dragon, Hiccup makes Toothless a red prosthetic tail fin.
Toothless loses part of his tail during the opening dragon attack, leaving him unable to fly normally. Hiccup gradually earns the Night Fury’s trust and designs a replacement fin that allows the dragon to steer and stabilize himself in the air. The red fin becomes one of the most recognizable visual details in the first film.
The prosthetic is not a complete replacement for Toothless’s missing tail. Hiccup must operate it, which is why the two become a flying team: Toothless provides the power and instincts, while Hiccup controls the fin through a linked mechanism. Later versions improve the design and allow Toothless greater independence.
This detail is sometimes confused with Toothless’s black scales or the red saddle and equipment associated with other dragons. The tail fin itself is red in the original animated film.