Here's a good sized list, so bear with me! (I know how improbably almost all of these fish are. These are all VERY BIG ifs!)
SALTWATER
- Any kind of marlin (dreaming big)
-Great barracuda
-Queensland grouper
-Shortfin mako shark (again, big)
-Yellowtail (not the tuna)
-Shovelnose guitarfish
-Great white shark
-Oarfish (bet you didn't see that coming!)
-Black tip reef shark
FRESHWATER
-Arapaima
-Silver arowana
-Some sort of giant catfish (piraiba, goonch, redtail, etc.)
-Lungfish
-Payara
-Goliath tiger fish (any tiger fish, really)
-Tiger shovelnose Catfish
-Lima shovelnose Catfish
-Any freshwater stingray
And anything else I might be missing!