I was just watching the G1 episode "Spike's Search" and can report that Danny is officially the first MLP character to use "pegasi" as a the plural of "pegasus"!
He also used "earth pony", though he was not the
first person to use it. (That would be Baby Lickety-Split in the movie, I think.)
You know, it's funny, as a kid I didn't like Megan, but I really couldn't STAND Danny or Molly . . . but these days I don't mind Danny or Molly. (I still can't stand Megan.)