Cognizant Credibility (Illusionist-related feat)

[Zom B]

Prerequisite: Spell Focus (Illusion)
Benefit: Whenever an illusion of a creature you created with an Illusion (Figment) spell is attacked, you may make a Knowledge check (DC 10 + monster's HD) related to the type of creature that you have created. If you succeed, the Figment uses the monster's armor class against that attack.
Normal: The Figment's AC is equal to 10 + its size modifier.

May be better as a Knowledge skill trick, come to think of it.

[bloodtide]

Looks kind of odd.

Would a spellcaster want the illusion to have the creatures real AC?

I'd say no.

Maybe you'd just want a Illusion AC boost. 'The caster can increase the AC if figments +1/CL or something'.

[Zom B]

You'd get any natural armor and dex bonus, as well as any other miscellaneous.

[Temotei]

May be better as a Knowledge skill trick, come to think of it.

I agree with this. As a mental skill trick, it fits its niche, doesn't take that much resource expenditure, and fits better than it does as a feat.

Possible prerequisites could be Knowledge (arcana), Listen, and Spot, or possibly even Spell Focus (Illusion).