There are a number of monsters in Bestiary III that lack the armor proficiencies they should have. As noted here: http://paizo.com/pathfinderRPG/prd/monsters/creatureTypes.html most creatures have proficiency in whatever type of armor they're described as wearing (eg. heavy, medium, or light) and everything lighter. If they are proficient in armor, they are also proficient in shields. I have listed the ones I have found below:
Asura (Agasura): Light, Medium, Shield
Cold Rider: Heavy, Light, Medium, Shield
Cyclops , Great: Light, Medium, Shield
Demondand (Slimy): Light, Medium, Shield
Demodand (Tarry): Light, Medium, Shield
Giant (Ash) Light, Medium, Shield
Giant (Cave) Light, Medium, Shield
Giant (Desert) Light, Shield
Humbaba: Heavy, Light, Medium, Shield
Kami (Zuishin) Light, Medium, Shield
Nephilim Light, Medium, Shield
Norn Light, Medium, Shield
Oni (fire yai) Heavy, Light, Medium, Shield
Oni (Kuwa) Heavy, Light, Medium, Shield
Oni (Void Yai) Heavy, Light, Medium, Shield
Valkyrie: Heavy, Light, Medium, Shield
Addendum:
Div (Akvan) Heavy, Light, Medium, Shield
Div (Akvan Prince) Heavy, Light, Medium, Shield
Akvan still needs heavy armor proficiency. However, I didn't see anything else wrong.