This one is, of course, a rip-off of that tired old line: That's no lady, that's my wife! (or ex-wife, depending on context). I thought it might be humorous to substitute the word lady with another random noun of my choosing, and sea turtle is as good as any, I suppose.
Other clever variations could be made to other trite phrases like He's not heavy, he's my brother, or ???
As a homework assignment, why don't you think of other random nouns that might go well in these and other phrases, and send them to me. I can't guarantee that I'll make a cartoon out of them, but it would be a good mental exercise...