You thought that land shark thing was a joke, didn't you? Nope. They are very rare outside of their southern coastal ranges, but a land shark was recovered as far inland as Pittsburgh. Biologists suspect it was a pet released into the wild when it got too large to handle.
Grass carp are harmless to adults, but large specimens may pose a threat to smaller pets and toddlers. Grass carp prefer well-maintained lawns, so parks and lawns with crabgrass and dandelions are safest.
Dandelions, clover and other common flowering plants will attract the lovely, tiny blossom minnow. Look out for spawning season in late April, when shoals of blossom minnows sometimes writhe their way across suburban flowerbeds.
Even the most urban dweller can keep their eyes open for the asphalt sunfish. This common, hardy fish shimmers an attractive blue-green and makes good eating. (Baltimore and DC residents are advised not to eat locally-caught asphalt sunfish due to the risk of elevated lead levels.)