>>42781916
Good point. I mean, I haven't studied the origin of that phrase, but I assume someone is handy because they are dexterous with their hands, meaning they can do all kinds of things. Ponies however don't have hands, and hooves just can't be dexterous in the same way hands can be. Magic can be, however. A unicorn's horn could manipulate things in all kinds of dexterous ways, making them very capable and helpful.
I believe a capable and helpful pony could be said to be horny rather than handy.