"Hell Horrors are the personal servants of other creatures. They circle their opponents at high speeds before attacking."
- Monster summary(src)
Hell Horrors are a Skeleton type enemy.
Diablo I
Hell Horrors are a Skeleton type enemy that appear only in the PlayStation version of Diablo I. They are found on Level 16, taking the place of the Blood Knights found in the PC version. Sir Gorash also appears as a Unique Hell Horror in this version of the game.
Diablo II
Hell Horrors are a cut sixth skeleton type which was not included in the final version of the game. They were supposed to appear somewhere in Act IVTemplate:Cite.
The Awakening and Diablerie has a Hell Horror after Horror and there is an unused color palette for the skeleton that looks like a red Horror. It's unknown if they would have Archers or Mages.