That's your own personal headcanon. There has never been anything stated in the show about Celestia and Luna's mother, much less her being named "Galaxy". Faust technically is Celestia's and and Luna's mother since she created the characters.
The fact that nothing about Celestia's and Luna's parents has been officially stated by the actual creators of the show means that while I'm sure you like the idea of their mom being a different alicorn named Galaxy, it is not official, nor is it true.
If you saw something about that, it was most likely a fan's own personal theory, no one knows for sure who Celestia's and Luna canon parents actually are. I hope that might have given you a bit more information on what is actually true regarding Celestia's and Luna's parents.
Winged Unicorns / Horned Pegasi have several different names and MLP:FIM doesn't own this idea. Does anyone remember the Cockatrice creature introduction in MLP:FIM? That creature was supposed to be half dragon, not half snake. So, that's two cases in the "what should we call it" department.
But really, why do you people care so much about this? MLP:FIM proved they don't care about what they're called, since they brought up Another name for these modern mythological creatures, so, pardon my french, but why give a shit?
That didn't sound right. At "MLP:FIM proved they don't care about what they're called", what I meant to say was that MLP:FIM proved they don't care about the winged unicorns being given a single, universal name.
Lauren Faust herself uses and prefer the term of Alicorn, which she occasionally refers to when talking of the Princesses... This, here, is one example : [link] And also, storyboard artist Sabrina Alberghetti : [link]
And, in any case, whatever use for the word that dates from over 400-500 years ago was... Now, in this 21th Century, and within this series, *Alicorn* IS the term used to designate the Princesses, by Faust and by a vast majority of people.
Whatever someone ''prefer'' the use of 2 words cut in half and lazily pasted together by a random person at some point in time... is that person's choice. An 'individual' as the right to choose whatever her/she wants... The 'majority', on the other hand, agreed on another term. That is all.