It's a Fairly Common issue with all Gear that have Special Effects...Jarl's/Divine/Wicked Gaze even Halos and Horns or Gears That utilizes Physics like Capes and Robes.
@nitocris83 Yes. below the person described in more detail. most likely this is due to the fact that the game considers horns to be a cloak from a technical point of view.
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it affects all mounts ingame.