The Fek'Ihri were an ancient enemy of Kahless the Unforgettable on Qo'noS 1,500 years ago, when he was building the Klingon Empire. Kahless conquered the Fek'Ihri wielding the Sword of Kahless, the first bat'leth. (DS9: "The Sword of Kahless")
In the storyline of Star Trek Online, the Fek'Ihri were the footsoldiers of Molor during his rule of Qo'noS, and banished with him to Gre'thor; they seem to be similar to Fek'lhr in appearance. Klingon players who reach the rank of captain must fight off a Fek'Ihri invasion of the Klingon Empire. It is hinted, though not explicitly stated, that they may have been artificially created by the Hur'q.