Pathfinder Adventure Path, Starfinder Adventure Path, Starfinder Society Subscriber
Sounds cool, i think someone would have to communicate to the horse.
To explain why it should drink something that doesn't taste as good as toxic waste. To explain that it should just accept the strange feelings it would experience while the potion is taking effect to avoid having to make a fort save. To explain to the horse that it is now able to run up walls etc. Oh and then having to deal with all the problems involved in upside down horse riding. I think you would need an oh-so sympathetic druid (and a Large Roll of Gaffa Tape).