Glorfindel

Glorfindel is an Elf from J.R.R. Tolkien's The Lord of the Rings. He meets Aragorn and the Hobbits as they are fleeing from the Nazgul, and takes the critically wounded Frodo to Elrond at Rivendell. He rides a white horse named Asfaloth.

In The Movies his role (and horse) was given to Arwen. This caused ... displeasure ... to many members of the Tolkien Fandom. In the horrendous Ralph Bakshi rotoscoped cartoon adaptation, his role (and horse) were given to... Legolas. This perhaps explains why he is the founder of SICTNIM (the Society of Important Characters Though Not Appearing In the Movies), A Glorfindel is known to have slain a Balrog centuries before the events of the Lord of the Rings. Whether or not it is the same Glorfindel is not known, as Tolkien himself never said anything definitive on the subject. As with Legolas, it may have started as a case of re-using names from the Lost Tales/Silmarillion in Lord of the Rings, and then having to retcon backstory to explain the names.

This results in two versions within fandom: As with the "Do Balrogs Have Wings" debate, we presumably shall never know, unless Christopher Tolkien unearths an unseen manuscript settling the question. This, however, is unlikely.
 * the "One Glorfindel" theory, where Glorfindel of Gondolin is rehoused by the Valar and returns to Middle Earth as a special agent of the Valar; Tolkien himself speculated about this as a theory, noting that it would have to occur in the Second Age and was perhaps like an early version of sending the Istari in the Third Age.
 * the "Two Glorfindels" theory, where Glorfindel of Rivendell is simply named after Glorfindel of Gondolin.