He should just be a crocdile.
Not even, like a giant talking crocodile, just a regular crocodile who just happens to be at the wrong place at the wrong time. There's a bank robbery and the crocodile just happens to crawl in there. Nobody realizes he's just a crocodile so all the civilians are like "OH NO IT'S KILLER CROC!" and the robbers are like "LETS GET OF OF HERE!"
Then the crocodile's just laying there underneath the skylight soaking up sun when Batman shows up. They wrestle for 10 pages and send him to Arkham and keep flushing him down the toliets.