I've tried everything Poggy... walking, trotting, standing still, even off the horse etc. Nothing I do gives me the option to feed the horse and elixir. The horse elixirs are greyed out.
When the horse's stamina is low due to long range galloping the need to feed it with a horse elixir is essential in order to replenish the stamina bar and fortify the horses vitals. It's not possible which must be an oversight or game glitch as they are clearly meant for this purpose.
If anyone can find out how this is done we'd all be very grateful !!
Rockstar... have you forgotten this feature in the game menu?