Long, long ago in the world of My Little Pony, the deer first mastered magic. It was a momentous occasion, a true milestone in the world, for it was the very first time any species had learned to harness the supernatural forces that infused the air around them.
There was just one problem: half of all deer couldn't use magic at all. The deer learned to wield magic by channeling etheric energy through the tines of their antlers, but female deer did not grow antlers, so they were not able to use magic.
This greatly upset the female deer, and they pleaded with The Wills That Draw The World, the supreme, ultimate creators of all existence. The Wills were silent, because as numerous as the does and cows were, their voices were not as loud as their male counterparts. So the female deer devised a new plan: they ceased mating and lovemaking with the male deer until they too could wield magic.
Now the bucks and bulls joined their female counterparts in pleading with the Wills, sending up prayers, hosting ceremonies, and simply complaining towards the sky. At last, the Wills responded. Like ink being drawn across a page, antlers sprouted from all the heads of the female deer. Even infant does and bucks grew tiny nubs. And when that generation of females had their first fawns, they too grew antlers, regardless of gender.
So it is that, in modern times, all deer, male and female, grow antlers. Therefore, all deer, male and female, can use magic- and indeed, some of the greatest magicians in the history of the deerfolk have been female.