i figure that when the original crystal princess or queen was alive there were some royal guards around, but when sombra came to power he eventually got rid of them. (also, he apparently tailored their armor to fit his style better ;) )
for the record, i do actually think that the crystal ponies are their own race without unicorns or pegasi, but for this comic, i need them. and on the other hand, why would ONE of them be a unicorn that rises to power, and no-one else? except if sombra came over and was like this kingdom is ripe for ruling. hmm. maybe a mixture of bothyes.”