You could call it Marni's Marvelous Mechanical Horse (or MMMH, lol). It would be a cyborg horse. Posted some examples I found online. I don't own any of the images posted.
Edit: You could come up with a series of Marni Trials we have to go through to obtain it.
Something like this as barding would be neat. It would fit within the black spirit theme, like the third one but black.
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