Appears that you have a counterstamped copper coin, possibly a U.S. Large Cent from the early 1800's. The stamp seems to be MIER, from what I see. As I recall, these were used much like early advertising tokens, redeemable at the particular business stamped upon the coin.
I agree with CC Hunter...counterstamped large cent. Ciudad Mier is a Mexican town just across the Texas border, although I don't know if that has anything to do with it. Nice find!