The owner of a local petstore had a Jack Russell named Murphy who would attack any dog that was more than a tiny bit larger than he was. He wasn't kidding around either. My Pit mix Gus was on the leash, and I was taking him inside for a visit, as it was one of his favorite places to go to. We were just going in and Murphy, who had never even seen Gus before, attacked him. Gus was startled and just stood there, and Murphy is yanking hunks of hair out of his neck. I'm trying to back out the door, because Gus is showing signs of getting angry, and Murph's gonna be hamburger if he's not gone, ASAP. Finally, after what seems like a long time, some kid grabs Murph and we get the door in between them and the kid carts Murphy back to the back of the store, snarling the whole time. After that, anytime we came, they had to lock Murphy up in the back room, as he was foaming at the mouth as soon as he smelled Gus. He kept wanting to get into it with Gus, even after a female black lab broke several of his ribs after he attacked her.
My present dog, King gets into trouble with other dogs because he wants to play, and doesn't care if the other dog wants to or not.