I seem to run into players who's service tag seems to be the randomly generated one you get when you play the game for the first time.
Although, I did play a match with someone using PONY, which happens to be one of the two ones I use, when I was using LUNA, my other one. I believe he asked me if my tag was a reference to something that GameFAQs users have been known to hate on (well, the TF2 board anyway), but I couldn't tell because the ingame mic is kinda hard to hear in the pre/postgame lobby, especially when not using a headset.
"You're dead if you aim only for kids. Adults are only kids grown up, anyway." - Walt Disney