Tenth Walker

The Tenth Walker is a type of Mary Sue found in the The Lord of the Rings fandom. It is a subspecies of the type of Sue that joins the canon heroes in their work, mission or quest. This particular Sue inserts herself (or himself, but females are more common) into the Fellowship of the Ring and travels with them. These fics tend to follow a distinct set of rules.

Girl from Middle Earth
The Sue is an 'ordinary' Middle Earth girl who happens to have an extraordinary talent. She could be a great swordswoman, a healer, an archer, have psychic powers or any other ability the author deems to be cool. She joins the Fellowship and becomes indisposable to them.

Girl falls into Middle Earth
The Sue is an 'ordinary' girl from the real world who somehow falls into Middle Earth. Often she has read the books in real life or otherwise predicts events that are going to happen. She often changes key plot points such as saving Gandalf from the Balrog or stopping Boromir from dying.

Elf
The Sue is an elf. Elven Sues are beautiful, wise, talented and graceful. They have names that are meant to be Elvish, but may be made up or taken from words used in the books. Elven Sues are noted for not being Elf-like at all, as they tend to have teenage mentalities that reflect the author's own personality.