Unicorns would probably have an okay time, so long as they TK the proper points of contact. Otherwise, I'm pretty sure the only guns ponies would be able to use would be some magic-fueled laser gimbals, kinda like what the ponies in this pic have mounted on their chestplates.
The idea's pretty interesting, honestly. I'd imagine horse warfare — proper
warfare, not the silliness in Over a Barrel
— would be built entirely around holding defensive positions and slowly building up defenses until they roll over the enemy, due to the fact that earlier magic-based weaponry probably wouldn't be able to track what the operator's looking at, and as such, they'd be stuck in a fixed position like some self-propelled guns in real life. And, well, you know how SPGs are used in pretty much any WW2 film.
TL;DR Unicorns are a yes, pegasi a maybe depending on how strong their feathers are, and earfies are a hard no outside of 'stick these to the sides of your body, orient yourself toward the enemy, then chomp the bit to make the bad guys go away'.