Magical Beast (Extraplanar)?
Combat: Like dogs and wolves of the Prime Material Plane, shadow hounds attack in packs using their powerful jaws to drag a prey off of its feet. Once down, the hounds swarm over the prey and devour it mercilessly.
Trip like wolves, or even a lesser form of savage like brachyurus?
In shadowy conditions (Plane of Shadow, moonlight, etc.) a shadow hound is a terrible foe. It can strike and then blend with the shadows so that it is 40% unlikely to be seen.
Shadow blend and hide in plain sight?
However, in bright light it loses 50% of its movement and cannot use shadows for concealment.
A variant of light vulnerability?
Regardless of lighting, +1 or better weapons are needed to harm these beasts.
DR 5/magic?
Ability scores of similar creatures...
Riding Dog (M): Str 15, Dex 15, Con 15, Int 2, Wis 12, Cha 6
Hell Hound (M): Str 13, Dex 13, Con 13, Int 6, Wis 10, Cha 6
Shadow Mastiff (M): Str 17, Dex 13, Con 17, Int 4, Wis 12, Cha 13
Spectral Hound (M): Str 15, Dex 14, Con 13, Int 2, Wis 15, Cha 8