In the episode Sonic Rainboom, right near the start you hear Rainbow Dash refer to herself as a 'Billy'?
The only reference I can find to this is a 'male goat'.
Why would she say that?
It's actually "filly" (a young, female, horse):
TWILIGHT SPARKLE: What's a sonic rainboom?
PINKIE PIE: You really need to get out more. The sonic rainboom is legendary! When a Pegasus like Rainbow Dash gets going soo fast... BOOM! A sonic boom and a rainbow can happen all at once!
APPLEJACK: And Rainbow Dash here's the only pony to ever pull it off!
RAINBOW DASH: It was a long time ago... I was just a filly.
PINKIE PIE: Yeah, but you're gonna do it again, right?
RAINBOW DASH: Are... you kidding? I'm the greatest flyer to ever come out of Cloudsdale! I could do sonic rainbooms in my sleep.