UPDATE (13/05/25, 12:30 PM GMT): An Oblivion player with the handle Famous-Escape-2491 over on Reddit has taken their own shot at mythbusting the spectral horse, and has found the combo of spells you've got to slap on an animal in order to get the visual effect shown in TaricIsNotASupport's clip.
UPDATE: I see your Oblivion Remastered ghost horse that now looks like a happy accident or Daedric-tier trolling, and raise you this totally invisihorse I netted while investigating it
Source: VG247 (opens in a new tab)