Regarding the issue of believeability I stand somewhere between K1ngcat and Robb. I view stories about characters with supernormal abilities as fantasies. A fantasy author is "allowed" to base stories on one or more impossible concepts without explaining them. For example you can introduce a fire-breathing dragon without inventing an explanation for how a dragon can breathe fire. In my opinion a successful fantasy spells out the nature of the magical elements and the rules by which they operate. Over the course of the story magical events should remain consistent with these rules. Everything else operates according to real-world logic.
If I were writing Dr Hypno my magical base would be "the doctor has the ability to transfer his consciousness into an animal and take control of its body." Then I'd make a list of questions to work out the attributes and limitations of this ability.
Does Hypno's mind replace the animal's consciousness, suppress it, or work alongside it? Some aspects of the animal's mind must carry through or else when he entered a dog Hypno wouldn't know how to bark or how to wag his tail. Is the animal aware that it's being controlled? Does Hypno have to struggle with the host's natural traits--aggressiveness, fear of water, etc.--which may assert themselves? Can Hypno inhabit only larger, warm-blooded animals, or can he take over reptiles, fish, insects? Worms? What is going on with Hypno's body when he's in an animal (personally I favor a vegetative state)? If his host is killed does Hypno's mind revert to his body or does his consciousness die along with the animal's? Does the animal remember that it was controlled by Hypno? If so, does it resent the intrusion? Is there a range for mind transference? If Hypno's body is on the other side of the world does his mind still return when the hypnotic effect wears off? Does the effect have a specific duration? Does this duration depend on the nature of the animal? Is it harder to maintain control over a more intelligent animal like a dolphin than it is over a flea?
You get the idea. Once I'd answered these questions I'd have guidelines to determine whether a future story would work. For instance I've decided that Hypno's mental control can not change the physiology of the host animal. He cannot make a cat's tail hold a pencil. He cannot make an elephant talk. Conceivably the good doctor could use a hypnotic trick to make someone think an animal is talking. But that's the same thing--inventing a new superpower in mid-story. I don't feel that's fair to the reader. Besides, Dr Hypno as originally conceived offers plenty of story possibilities.
Having said all this I acknowledge that this is all hindsight by an old coot writing 80+ years after the fact. I'm not a twenty-something cartoonist puzzling out how to do these newfangled comic book thingies. Who knows what Frank Thomas thought of the character, or of his job, or of Art in general? I'm basically playing mind games for fun.