Agouti restricts black and only black. It causes a black horse to become bay, brown, or wild bay. Any horse that is further modified from bay, brown, or wild bay will have whatever form of agouti that caused the original modification of black (ie, buckskin or perlino are still A_)
A red horse is homozygous red (ee) and has no black to be restricted. So a chestnut/red dun/palomino/cremello/what have you CAN carry agouti, but you'd never know just looking; it would require a genetic test.
A "normal" dun is actually dun on bay, so yes a bay dun would have agouti because bay is black + A. A red dun COULD have agouti, but it is red-based and therefore no black to restrict. Dun is a separate gene altogether that dilutes color, lightens the body and not the legs, mane, or tail, hence why red duns and grullos have a similar darker leg effect that agouti would cause separately.
Last edited by Poseidon; 03-23-2013 at 08:10 PM.