Thorn was a red dragon in the Inheritance Cycle by Christopher Paolini. Thorn is very young, younger even than Saphira, and is very inexperienced. His body has magically altered to be much bigger than it should by Galbatorix so that Thorn and his Rider, Murtagh, can be formidable enemies for Eragon and Saphira. Thorn and Murtagh are not in the service of Galbatorix by choice, but were forced to swear oaths of loyalty to him in the "Ancient Language", meaning they cannot break their oath. It was through Thorn and Murtagh that Galbatorix was able to slay Oromis and Glaedr, leaving Eragon and Saphira as the last free dragon riders in Alagaësia. Durin the battle at Uru'baen, Murtagh was able to change Thorn and himself through the former's love for Nasuada. Thorn rebelled against Galbatorix, and after the battle, flies north with Murtagh to cool off all the rage he has built up.
It is noted that Galbatorix's magic has made Thorn go insane.