Wasn't this thing inn a Spy Kids movie.
oh my god 
YES
YES
The Slizzard from the second Spy Kids movie was similar appearance-wise, but I they aren't the same thing, the Slizzard was a hybrid created in laboratory and the Tanystropheus is a natural creature.
Oh, Tha explains it.
I don't know.
There was a similar creature in the second Spy Kids, it was a creature called Slizzard, it had a lizard body and the head and long neck of a snake. But from what I saw of it in the movie, the Slizzard is more aggressive than a fisheater and it would prefer to hunt warm-blooded creatures rather than fish.