‘Midnight Whistle’ is what her mother told you as she was slowly passing.
 
“Don’t worry, ma’am, I’ll protect and raise her as if she was my own.”  
“Thank you…”
 
You hardly understand what you were getting into at the time, just wanted to keep her spirit up in her final moments.  
Now, sitting at your family’s spring in the Everfree, you look down upon the sleeping form of that very filly you took in.  
“If brother can easily raise Star, I shouldn’t have any problems with being this child’s mother.”  
(A sweet little snore erupts from the little zeeb, you smile.)  
“Hmmph… Sweet dreams, Midnight, may you sleep well, tonight.”