You can mount a house when you are polymorphed and the horse just seems to disappear, but really you gain the speed of being on the horse and if you click yourself like you would to dismount, the horse reappears!
Perhaps consider making mounting the horse impossible if your polymorphed?
Anyhow, its a good way to smuggle a horse.
Thanks!
Mindless Automaton
Linux - UOX3 - 0.99.5 dev branch
Win10Pro 19042.572 - UOX3 0.99.3a; Razor 1.0.14; Client 7.0.87.11 or 4.0.11c (Patch 0)
Unable to reproduce. When I use the polymorph-spell to polymorph into something or other, I get the following message when trying to mount a horse while under the effect of the polymorph:
"You cannot ride anything under polymorph effect."