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")
Background information Edit
The specific spelling comes from the episode's script. 
In Star Trek Online, it is established that the Fek'Ihri were the footsoldiers of Molor during his rule of Qo'noS some 1,500 years ago, and banished with him to Gre'thor; they seem to be similar to Fek'lhr in appearance. Furthermore, it is revealed that they were originally created by the Dominion 2,000 years ago in an attempt to create a soldiers caste by genetically manipulating Klingons, using creatures from Klingon mythology as inspiration. As the plan to control them via the addiction to ketracel-white failed, the Founders then turned to the Jem'Hadar, who would become their front line soldiers instead.