The dog attacked the guinea pig! The guinea pig was on my lap, and my dad was sitting next to me with the dog on the leash. My dad let the dog sniff the guinea pig, because they need to get used to each other. Then all of a sudden, he bit him! However, my dad was fast enough to prevent him from really sinking his teeth in the poor little creature, and he only got his fur with his front teeth. He did swing him around however, until the fur was pulled out of his skin and he fell on the ground. The dog kept trying to get him, the guinea pig kept trying to escape, I kept trying to grab the guinea pig, and my dad kept trying to hold the dog. Eventually, my dad managed to take the dog outside, while my mom and I managed to grab the guinea pig and put him under a towel (that gives him a feeling of safety). He was scared as hell, and there's an open patch where his fur was yanked out, as well as some tooth marks. Fortunately, he wasn't bleeding, however he was seriously stressed out (guinea pigs are highly sensitive to stress, and it can even kill them). I managed to calm him down, while my dad was outside being seriously pissed at the dog.
We've now decided that the two won't get used to each other (maybe they will, but we're not gonna try again), and we're still thinking of a way to solve this problem, since the guinea pig is currently staying on my room, but he can't stay there forever, and since he was here first, we're not gonna get rid of him.
The image of the dog swinging the guinea pig through the air is still engraved in my mind. It was a horrible sight.