"Pony" is what the unwashed masses call a P-51 Mustang....
Any serious student of WWII aviation would rather apply a hot steam iron to their testicles before using that term.
In WWII the generally used name was P-51. Some pilots did use the term "Mustang", but no one muttered the sobriquet, "Pony". It's the aviation equivalent of saying "dude" rather than a proper name (skillkill, take note).
So, if you want to sound like you have some knowledge of the WWII aviation history, avoid the term "Pony" and use P-51 or Mustang. In an online game, perception IS reality.
My regards,
Widewing