Batman lives a rather solitary life outside of Alfred, but in the newest Batman Annual, Bruce gets a new addition to the household.
Spoilers incoming for Batman Annual #1, so be warned.
Some of the most devastating moments of Batman's career have come at the hands of The Joker, and the book starts out with another instance of his long-running cruelty. He liked to have wild dogs around dressed up in various playing card themes (Queen, King, Ace, etc), but after tiring of the joke, he left the four dogs in a ditch and left them to their own devices. Once dog entered full survival mode, and killed the other dogs, though it also could have just been part of the "Aces High" joke. Either way, far from pleasant circumstances.
The dog is taken to the pound, but Alfred isn't one to give up that easily, so he gets the dog from the pound and brings him home. The dog and Bruce don't exactly get off to the smoothest of starts (he does stand on his dining room table to be fair), and as time goes one it seems that Alfred is running a fool's errand in trying to get him trained. Still, as the weeks and months go on, he persists in his training routine to comedic results.
Bruce isn't convinced, but one fateful night in December he returns to the cave nursing a few wounds from the battle earlier in the evening. A certain furball senses his presence and starts to fawn over him, licking his hand and bringing Ace full circle. Later that month he even gets himself a mask for Christmass, and Bruce asks "What do you say, boy? D'you want to be a Bat-Hound?"
It's a lovely story, one of three in the Batman Annual, and you definitely should give it a read for the full effect.
Ace the Bat-Hound has had various forms over the years, but he originally debuted in Batman #92 in 1955. He was created by writer Bill Finger and artist Sheldon Moldoff, but there is debate as to if he was modeled more after Rin Tin Tin or Krypto, who debuted earlier that year in Adventure Comics #210. He's typically depicted as a German Shepard, he's been relatively absent from recent books, and this marks a new origin for the character.
That said, he has been present in many forms of animation, from Batman Beyond, Return of the Joker, Krypto The Superdog, and Batman: The Brave and the Bold.
Batman Annual #1 is in comic shops today. You can check out some images of Ace in the gallery.