You don't give any details about how old, or big the animals are, or who was there first, but the general rule about fighting is territory. Cats and dogs are territorial animals, and that's what the fighting is about - making sure both know that the other is the subserviant member of the pack, which needs to be reinforced on a regular basis! To prevent this, try to encourage each animal to have a specific part of the house as their own. Put individual bedding and food bowls there, and keep the other out while the 'owner' is feeding or sleeping. Eventually, the natural boundaries will be recognised, and with a specific piece of house area to call their own, the two are likely to respect the other's privacy, and feel less inclined to force the superiority angle so much. It's worth a try, but be patient.