Everything is designed and made for creatures with opposable thumbs.
I understand it's a kids show, and it's just easier and more relatable for the young target audience if a hammer looks like a hammer they are used to seeing.
Why would a pony's version of a hammer have a handle? Teeth should shatter if a hammer was used with the mouth. (go ahead and try it, i dare ya)
I was doodling on paper and came up with a logical design for a pony's hammer. It's essentially a metal block with a strap, that can still be walked on. (our hammer is basically a metal block at the end of a stick) Horses already have the leverage advantage when it comes with swinging their limbs around.
(seriously this is a dumb and awkward tool in their universe. it's going to poke her eye out.)