As his name would suggest, Ace the Bat-Hound is Batman's dog. Well technically Ace is Bruce Wayne's dog, and the Bat-Hound is Batman's dog, but you get the idea.
Originally, Ace belonged to an engraver who was kidnapped by criminals hoping to use his skills for counterfeiting, and Batman used Ace's nose to help track his owner down. When the engraver was rescued, he took a new job which left him no time to care for Ace, and Bruce Wayne offered to adopt the dog. To keep Ace's distinguishing marks hidden when palling around with Batman, he gave Ace a Batman-style hood.
In modern comics, Ace has basically disappeared, since he's considered to be too campy for current readers.
Pros: A dog in a Batman mask is pretty awesome, in a Chris Sims sort of way.
Cons: A dog in a Batman mask is pretty silly, too. And I think that would earn protests from PETA.
How I'd do it: Give Bruce a dog, call him Ace. No mask.
Casting Guesses: Ace was a German Shepherd (and I'm pretty sure a ripoff of Ace the Wonder Dog), so... some German Shepherd.
Verdict: Possible, if it's just Ace the dog, but no chance for Ace the Bat-Hound.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.