Tolkien's letter #180

[Ophelia]

There is hardly any reference in The Lord of the Rings to things that
do not actually exist* on its own plane (of secondary or sub-creational
reality): sc. have been written.

Footnote:
*The Cats of Beruthial and the names of the other two wizards (five minus
Saruman, Gandalf, and Radagast) are all that I recollect.

I have been having troubles with understanding this quote. What I persume Tolkien has said here is that only the Cats of Beruthiel and the names of the two other wizards have references in the Lord of the Rings... This really puzzles me because I am not really sure that I have understood it right.

Your explanations are very welcome.

NOTE : This is very urgent for a great cause !

-Ophelia-