Fek'lhr:
A mythical Klingon beast that was the guardian of
Gre'thor.
- Star Trek Encyclopedia
It is not to be confused with the devil equivalent in terran mythology. Fek'lhr (AFAIK) has only been mentioned in one episode (Devil's Due of Star Trek: The Next Generation).
There is (or used to as their website is no longer existant) also a KAG in British Columbia, Canada named the IKV Fek'lhr's Fury.
In the Klingon Dictionary, the Klingon romanicized spelling of Fek'lhr is veqlargh.
See also: Gre'thor