I don't think it's Serrano, not tapered enough. Not Fresno, either, since those have a more triangular shape from the stem down to the point. Also, not Jalapeno, since Jalapenos have gentler contours and more blunt and rounded end at the bottom. Definitely not a Thai, either, since those are very narrow and come to a very sharp point on the bottom. It is a very generic looking pepper, hard to tell exactly what it is. Most likely Annuum species, though it could also be Frutescens, too. Good luck in your quest. Try a taste and tell us what the flavor and heat are like!