It is a Gambusia, a Mosquitofish. You can usually spot them by the black bar below the eye, it's a bit washed out but visible. They often get mixed with feeder guppies; in fact, they sometimes get shipped AS feeder guppies.
They are very aggressive for a livebearer, and will attack most smaller fish. I would not keep it with the guppies for too long.
Ah, I definitely agree on the Gambusia assessment. Having a good look at pictures, she must be one. Not as pointed in the face as some of them, but the other markings are spot on.
Gambusias tend to be aggressive and therefore fin nippers. If you want to keep your guppies fins intact I would remove her. Nothing against Gambusias, I collect them all the time.