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")
The specific spelling (with a capital I) 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 improve upon the then-recently created Jem'Hadar by genetically manipulating Klingons, using creatures from Klingon mythology as inspiration. However, the Dominion was unable to control the Fek'Ihri through addiction to ketracel-white, so the Founders abandoned the experiment and settled on the Jem'Hadar as their warrior caste.