Maybe, but it doesn't go boom and shoot pieces of metal and make lots of noise for the horses.
I think that military horses had to get special training to cope with gunfire and the noise that accompanies it.
yeah, for a decent firearm you will need rifling and a projectile that conforms to certain aerodynamic requirements.
I think the earliest is the Mine (fun aside, sounds like the Bulgarian word for handjob, and handjob balls I shall call them.)ball, which was at least Napoleonic era and I doubt...