I use Oblivion Reloaded so I have to disable the ability to cast spells on horse in the other mod. The only thing is, I forget which mod was that I verified that it's not Trifle. But I do remember it's a mod made by some very reknown modder.

Any ideas what is the mod's name?

That would probably be http://www.nexusmods.com/oblivion/mods/42658/?.

UV, DR and Migck's Miscellaneous add the cast on horse features.