RB? No.

Mammal? Strictly speaking, I think the answer has to be no. *some muttering in the audience*

Disney creation? No.
Aquatic? No.

Summing up: we are looking for a fictional, carnivorous, single specific animal, which is in part a newspaper cartoon character, and in part an animated cartoon character (we don't know if those are the same parts) and in part comes from written fiction, though "'in writing' is not how one would typically describe its first incarnation." It can be interpreted symbolically, though that's not necessarily helpful, and it is not mythical. Strictly speaking, it's not a mammal (though the audience muttered about that answer). It's neither a Disney creation, nor acquatic.

Was its first incarnation in the 19th century?

[Irouléguy] Excellent summation. Minor clues: I would not put too much emphasis either on newspaper cartoons or written fiction.
[Dandalf] Well, we can discuss that when the round is over. :-)
19th Century? No.
Lewis Carroll? No. *some smiles in the audience*
Adult audience? That's not really the target market, although many adults like at least some incarnations.
Hobbes? No.
A clue: When I set this AVMA I expected a question to which I could give a correct but misleading answer. And you fell right into my trap. Mwahahahaha!

Human form? Yes! *applause*

Superhero? Yes. *applause*

Superman? YES!

As for "mammal", my uncertainty was the following. As far as I know, Superman has all of the traits that are usually taken as defining a mammal. But I think that the strict definition of the term has an evolutionary basis, and thus cannot apply to someone not from earth. I expect to be corrected by either a biologist or the morniverse equivalent of Comic-Book Man from The Simpsons.

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