I think I'd really like you to flesh out the humongous dogs a little more (habitat, habits, diet, etc.). That, being something I've never heard of, is your best addition to this thread. And if you have other original works, by all means, type it up.
Ahem. Now let's say there are shape shifters that live amongst the races of the world. There is no real preference as to where they would live, nor what they eat; mostly they blend in with the surroundings of society they live in.
These beings are of a sentient race that have a high intelligence and are inherently adept at the following skills: stealth, acting, observation; and have the following inherent abilities: humanoid transformation (varies), and animal transformation, vegetation transformation.
Weaker, or younger, shifters will be able to shift into one or two of each ability type, while the stronger characters will have a variety of choices.
These beings not only live next to us, but also breed with those they are able to transform into; usually humanoid characters (though plants and animals are also used to spawn more). The offspring of these inherit the high intelligence and the skills and abilities of their parent, though these powers are inactive until puberty.
Guard dogs have a nose for shape shifters. They tend to growl and, at times, attack with out provocation. These dogs will not calm down until the presence of the being is gone.
Reaction to them is quite mixed. In the more rural areas they are feared, and at times, hunted. Within the larger forms of society they are more accepted, and even ignored; unless they use their abilities for fiendish acts. Rogue shape shifters can cause even their fellow shifter to hunt them down, to protect themselves.