She's definately a leghorn hen....I have quite a few of them, and their combs get huge! My hens just started laying, and their combs are so large they hang over their eye on one side. I have a little leghorn roo, and he started getting his comb at three weeks. Right now it's enormous! It was really cute at three weeks, he had a perfect little red comb and a tiny fuzzy/feathered body....LOL!!! If there's no one around who could own the hen (there's gotta be somebody, that chicken had to come from somewhere) go ahead and quarantine then see if you can get her integrated into your flock. Just keep her separate for a good month or two!!!! Make sure you ask around, cause it would suck for someone to have lost their little hen....
P.S. Could be a very young roo....it all depends on how old the chicken is....a laying age hen will have a huge comb, so about 4.5 to 5 months she could definately be a hen. Check the placement of her 'hip' bones and see if they've spread. If so she's a laying hen. If it's a roo, it would only be about 2-3 months with a comb that size, IMO. Any visible saddle feathers??? Can't tell from the pic...Otherwise I would DEFINATELY say you found a hen!