Mono Ghost may be better than Mono Ice, but IIRC Rapid-Strike Urshifu was liked more than Single-Strike mainly because of the typing and fear of Fairy types. Yet, Single-Strike proved itself to be be more useful.Man, watch the Ghost Horse, the least liked out of the two horses, be extremely broken somehow.
If the leaks are indeed true, then I think the Ghost Horse has more in-house competition with Dragapult, Gengar, maybe even Aegislash. It also doesn't help that if it's Special oriented, it could may as well be your standard Ghost distribution with weak Defense. Pursuit may be gone, but Sucker Punch and Knock Off could be an issue. Not to mention Chansey, Blissey being a pan in the ass for it. Potential coverage moves seen on other horses are also seem to be way more physical. Zen Headbutt, High Horsepower, Heavy Slam, Wild Charge, Play Rough, EQ, Rock Slide, CC etc.
Naturally, Ice Horse could face issues as well, but a potentially stronger STAB compared to Icicle Crash, or at least a similar base power one with 100% accuracy would be a blessing. Obligatory HDB also says hello. Maybe even AV could be viable, heck, with just usable Special Attack, Freeze-Dry for some surprises if it gets it.
So again, assuming the leaks are true, I lean towards Ice Horse, both in design and potential. Don't destroy me, GF. :"D