Horse might be ok.
I used to keep my GS and HP parked in the office garage, years ago, in Leman Street.
The HP was kept under a dust cover, and a fox got accustomed to shitting on it semi-regularly.
That's when I learnt that Fox shit smells much more (and worse) than horse shite.
I'm currently on a ferry floating across Bay of Biscay.
No horses or foxes (I suppose) but I left the helmet on the bike rear rack, secured with those metal wire thingies, and high enough to avoid being crapped on by said fox or horses (if present). But not sure about any potentially disgruntled crew, as they can defecate higher and/or with more precision. I guess I'll discover tomorrow evening.