Complex sentence: Hiccup realizes that the Night Fury cannot fly properly because of its crippled caudal fin, and gradually domesticates it under the name of "Toothless", for its retractable teeth.
Split into simpler sentences: Hiccup realizes that the Night Fury cannot fly properly because of its crippled caudal fin. Hiccup gradually domesticates the dragon and gives it the name "Toothless", for its retractable teeth.