the basic story is, that Luna is not able to produce own offsprings/childrens  
so she asked Twilight and the other alicorns to cast a spell that would create life.  
the goal was to only produce one child, but something went wrong and a second one got created (which is the one with the big curved black horn) this lead to the accidental created unicorn to be able to create wings trough magic, but the magic can only be absorbed from other unicorns (in this situation, she was absorbing magic from celestia which is displayed due to the colored cracks in her horn)
 
the other unicorn (with the wavy hair) has a kind of galaxy cloud as hair, it floats simular to the one like Celestia and Luna, just that its more cloudish like with partikels flying off and reconecting to her mane
 
Her magic is as strong as a alicorns magic, even tought she does not have wings
 
she also has a tail simular to the tails like from the movie ‘‘the last unicorn’’
 
 
Of course the pony folk does not like the idea of ponys or life created trought magic because its against nature. so where kept hidden from the other ponys till they arrived teenage hood to be able to deal better with the voice of the ponys. of course they are facing a lot of insults and mean looks from other ponys.
 
Twilight, the princess of friendship and now, principal of the school of frendship is very upset about the reaction of the citizens
 
tldr  
The ponys where created trough magic, and are seen as monsters due to they’re unnatural way of getting born